Fixed #120. Don't send invitation to unknown or existing user.
Fixed #121, reject users status marked other than 'invites'.
Split up api.py into multiple views files. Fixed #87.

In order for Flask to recongize all the new routes, we must import the modules at app.py level. I have commented out loading TESTING_PLUGIN for now to avoid circular import.
Fixed #123 (don't delete user when delete invite). Fixed #124.
Merge pull request #122 from yeukhon/issue87

Split up api.py into multiple views files. Fixed #87.
Merge pull request #125 from yeukhon/issue123

Fixed #123 (don't delete user when delete invite). Fixed #124 update user status on acceptance