User authentication and security_level restriction

A few different things in this PR:
- UserController with `login` and `logout` actions (routed to by POST `/login` and GET `/logout` respectively)
- Some basic views to test it out
- `authenticate_user` application level method. Call this from any controller action (or more likely a `before_filter`) that requires the user to be logged in (no security_level restrictions)
- `restrict_to(level)` application level method. Call this from any controller action to restrict to a certain security_level. The level argument is the string or symbol form of the enum: ie `restrict_to(:owner)`
- Add the mocha and debugger gems for future use

@oharaandrew314 @ryanseys