Authenticate Mandrill Webhook Requests

I added support for authenticating Mandrill's webhook requests using the protocol described here: http://help.mandrill.com/entries/23704122-Authenticating-webhook-requests

I bumped the version number (separate commit) to account for this improvement, and described how to use it in the README.

I did not add tests because I could not readily determine a test that would be sufficiently orthogonal to the functionality.