session/authenticate - Authenticate the current session.

Method parameters:
username - The username of the user to authenticate.
password - The password of the user to authenticate.
remember - Remember the session.

Returns a JSON document of the following format:
<boolean> - Always true.

Expected errors:
ENTER_USERNAME - The username is empty.
INVALID_USERNAME - The username is invalid.
ENTER_PASSWORD - The password is empty.
INVALID_USERNAME_OR_PASSWORD - The username or password is invalid.
USER_DISABLED - The user is disabled.
RATE_LIMITED - More than 120 attempts made from a client IP address in the last 60 seconds.