Changes:
- Updated Event to use Category IDs instead of Categories (to fix a duplicate category problem)
- Updated tests that referred to getCategory() method of Event to work with getCategoryId() method.
- Fixed a random CategoryModel test that had a typo and wasn't working.
- Fixed the Category equals method from throwing a null pointer exception on a null string.
Merge branch 'dev' of https://github.com/tomorrow-nf/wpi-suite-team5 into dev