Let better_errors work from behind my NAT
Add integration test for successful registration.
Add integration test for failed registration
Show errors on sign-up page
Implement sign up, sign in, log out functionality.

* Added database_cleaner gem
* Added sign in, log out to nav
* Changed sorcery config to use (downcased) email address
* Created Sessions controller and view for 'new'
* Integration tests for successful login and logout
Show error on failed sign in.

* Add flash[:alert] to application layout.
* Change URL for sign in to /sign-in
* Integration test for failed sign in.
Add Pledge model and unit tests.