Categories should have start and end dates

[ethanp]

Done

1. I have updated the schema for Category to have start_date:datetime and end_date:datetime
2. Fields are allowed to be NULL
3. The create and edit forms need datetime-pickers
4. categories#index will need a show all toggle like tasks#index does
5. The avg time spent method for Category is going to need to be updated to only divide by the time within the date range

   Todo

none left!