tests and a couple of updates

Hi there! First of all: **YES!!! THANK YOU!!!** I've been needing a "Dancer::Plugin::Auth::Twitter, but for Facebook" for a long time, and Dancer::Plugin::Facebook, the closest thing that existed, is currently buggy and unmantained :(

I took the liberty of making a few tests for the code, and also updating the code and pod to comply with some good practices in terms of Dancer session handling. I tried my best to comply with your project's code standards and to change as little as possible, and every change is a specific commit, so it's easier for you to review.

All tests pass, and I'm already using my updated fork in production (did I say "THANK YOU" already? You rock! :D)

Speaking of which, why isn't this on CPAN already? \o/

Thanks again for writing great software and keeping Perl and Dancer awesome! Please let me know if you have any questions, or if there is anything I can do to help get this to CPAN as soon as possible :)