Implement new features for cly.py 'event' command.

* The user can now create a new event with a not defined end date.
* The user can now edit the name and the start and end date of a given event.
* The software can now check if the event the user is creating or editing
  is overlapping another event already present in the database.
* The software will allow the shortcut keywords 'now' and 'none' in the
  'event set --end' command.

cli.py

@@ -78,18 +78,48 @@ def tabulate_events(events):
     return tabulate(tab, headers='firstrow')
 
 
+def get_overlapping_events(session, starts_at, ends_at):
+    q = session.query(Event)
+
+    if ends_at is None:
+        q = q.filter(Event.starts_at <= starts_at)
+    else:
+        q = q.filter(Event.ends_at >= starts_at)\
+             .filter(Event.starts_at <= ends_at)
+
+    return q.all()
+
+
 @event.command('add')
 @click.argument('name')
-@click.option('--start')
-@click.option('--end')
-def event_add(name, start, end):
-    start = datetime.strptime(start, "%Y-%m-%d %H:%M")
-    end = datetime.strptime(end, "%Y-%m-%d %H:%M")
-    event = Event(name=name, starts_at=start, ends_at=end)
+@click.argument('starts_at')
+@click.argument('ends_at', required=False)
+def event_add(name, starts_at, ends_at):
+    starts_at = datetime.strptime(starts_at, "%Y-%m-%d %H:%M")
+    ends_at = (datetime.strptime(ends_at, "%Y-%m-%d %H:%M")
+               if ends_at else None)
+
+    if ends_at and starts_at >= ends_at:
+        print("Could now add event: specified start date ({}) "
+              "is past the end date ({})."
+              .format(starts_at.strftime("%Y-%m-%d %H:%M"),
+                      ends_at.strftime("%Y-%m-%d %H:%M")))
+        return
 
     with db.get_session() as session:
+        events = get_overlapping_events(session, starts_at, ends_at)
+    if events:
+        print("Could not add event: another event is overlapping the date "
+              "range you have specified.")
+        print(tabulate_events(events))
+        return
+
+    with db.get_session() as session:
+        event = Event(name=name, starts_at=starts_at, ends_at=ends_at)
         session.add(event)
+        session.flush()
     print("Event succesfully added.")
+    print(tabulate_events([event]))
 
 
 @event.command('list')
@@ -103,6 +133,71 @@ def event_list():
         print("No events found.")
 
 
+@event.command('set')
+@click.option('-n', '--name')
+@click.option('-s', '--start')
+@click.option('-e', '--end')
+@click.argument('event_uid')
+def event_set(event_uid, name, start, end):
+    with db.get_session() as session:
+        event = session.query(Event).get(event_uid)
+
+    if not event:
+        print("No event found with id #{}.".format(event_uid))
+        return
+
+    if name:
+        event.name = name
+
+    if start:
+        starts_at = datetime.strptime(start, "%Y-%m-%d %H:%M")
+
+        if starts_at >= event.ends_at:
+            print("Could not edit event #{}: specified start date ({}) "
+                  "is past the end date ({})"
+                  .format(event.uid,
+                          starts_at.strftime("%Y-%m-%d %H:%M"),
+                          event.ends_at.strftime("%Y-%m-%d %H:%M")))
+            return
+
+        event.starts_at = starts_at
+
+    if end:
+        if end == 'none':
+            event.ends_at = None
+        elif end == 'now':
+            event.ends_at = datetime.now()
+        else:
+            ends_at = datetime.strptime(end, "%Y-%m-%d %H:%M")
+
+            if ends_at <= event.starts_at:
+                print("Could not edit event #{}: specified end date ({}) "
+                      "is before the start date ({})"
+                      .format(event.uid,
+                              ends_at.strftime("%Y-%m-%d %H:%M"),
+                              event.starts_at.strftime("%Y-%m-%d %H:%M")))
+                return
+
+            event.ends_at = datetime.strptime(end, "%Y-%m-%d %H:%M")
+
+    if event.starts_at and event.ends_at:
+        with db.get_session() as session:
+            events = get_overlapping_events(session,
+                                            event.starts_at, event.ends_at)
+        if events:
+            print("Could not edit event: another event is overlapping the "
+                  "date range you have specified.")
+            print(tabulate_events(events))
+            return
+
+    if any([name, start, end]):
+        with db.get_session() as session:
+            session.add(event)
+            session.flush()
+        print("Event succesfully edited.")
+        print(tabulate_events([event]))
+
+
 @cli.group('product')
 def product():
     pass