Json api

Changes include:
- Added JSON paths to `config.py`
- Added `_logout_user()` in `login.py` for handling logging users out (via destroying session cookies and `UserSession` in NDB datastore.
- Added `HTTP/1.1 400 Bad Request` error handler in `login.py`
- Added `json_object()` function to `User` class in `users.py`; will return a publicly displayable User object (i.e. what to return in JSON requests).
- `_login.json` via `JsonLogin` will: return information about a logged in user; log out a user; log in a user.

Todo:
- `JsonLoginPasswordReset`
- `JsonLoginPasswordForgot`
- `JsonLoginAcivate`
- `JsonLoginPasswordChange`
- `JsonLoginCreate`