Moved Category out of Post model

Category is now its own app, and can be RESTfully accessed in the
near future.
Adding Posts now adds categories and visibility exceptions

Use spaces to separate usernames and categories.  Intelligently
checks whether it needs to create a new Category object or use an
existing one to add an entry into PostCategory table.
Merge branch 'master' of https://github.com/awwong1/CMPUT410-Project