Add parent references to Answer and Comment.

This required making Comment a generic type.
I originally thought the circular references woudld break the Java GC,
but it looks like it will actually be fine.
Fix tests.
Made changes to main activity to store username once
Merge pull request #73 from stephenjust/updated-models

Updated models
Merge branch 'master' of https://github.com/CMPUT301F14T14/Project into UserAssign