Flick handlers take a 4th argument `done`.

When called, `done()` ends the GitHub WebHook handling and forward to the next express handler.
Deprecate the 'known' option of flick.whitelist().
Add flick.github(secret).

This helper checks if 'x-hub-signature' header is valid, and reject the request otherwise.
It should be used in place of flick.whitelist() when you want to make sure that the request comes from Github.

Closes #4
Add NodeJS v0.10 to the list of engines to run the tests against.
Update dependencies.