When attempting to login, the error messages should not leak information about whether a username is valid.

This also means the error message is more appropriate if somebody tries to login with somebody else's username and their password.
Update tests and documentation to reflect changed error messages with incorrect username or password on login attempt.
Merge branch 'master' into feature/limit_user_leak

Conflicts:
	users/lib/warden/strategies/secure_remote_password.rb
Merge pull request #36 from leapcode/feature/limit_user_leak

When attempting to login, the error messages should not leak information...