Implement UserMixin.

User collection and its functionality can be used in several
controllers, and for reusability they should be separated into an
independent mixin.
Unify response format of User and Session controllers.

The Unity game engine can't parse a response body of non-200 response,
and this patch implement a wrapper to form a response as a JSON object
containing a 'status' field.
Implement user login handler.

Until now a user's password hasn't been checked for login. This patch
implements the login functionality including checking a username and
password.
Implementing testcases for User and Session controllers.

Add testcases and helpers to write a testcase into
ApplicationSpec.scala.