Removed simple_authentication_token

For now, it has been replaced with the recommended Devise solution.
Added custom controller to Devise
Moved devise/sessions to tokens/sessions
Added a SessionController that returns an auth token.

This is a stopgap since it should inherit from its parent. Right now,
it is a carbon copy of Devise's SessionController except instead of
redirecting the user after log in, it shows their auth token in JSON.

This is 1000% a TODO.