Tweak example app
Write strategy as standalone class

This should make things easy for any unit tests.
Improve strategy

- use warden helper methods (redirect!, session, etc.)
- simplify logic
- clean up session after login
Add support for implicit callback path

When no default callback path is set, the request path will be used instead.
This simplifies the usage since no special endpoint for the callback needs to
be created.
Return to original url after authentication

This was not working previously.
Merge pull request #18 from fphilipe/improve-strategy

Improve strategy and fix redirect