Miscellaneous improvements

Improvements:
* Enabled support for using custom session controllers
* Fixed bug in test environment authentication
* Added realm to username when authenticating so users can just give their username
* Stopped trying to set password when creating user
* Updated readme

These changes will probably break existing apps that use this gem due to the change from authenticating by username and realm to just username. You may not want to pull these changes in, or if you do you might want to bump the version number. We found these changes were helpful for our use of this gem, maybe they won't be helpful for yours.