* The user can now create product categories with the 'category add' command.
* The user can now add products to categories with the
'product add --category INT' command.
* Product has a new 'order' column.
* The user can now specify the sort order at product creation time with the
'product add --sort INT' command.
* The user can now rearrange the products with the 'products set --sort INT'
comand.
* The user can now show a sorted list of products in cli.py with the
'product list --sort' 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.
