Slight refactor to consistently use the request passed as a parameter instead of pulling the request off the callback_host in one place.
Refactored Mandrill signature authentication to a separate method meant to be used as a before_filter.
Described refactored signature authentication syntax in README.
Changing check from `unless head?` to `if post?` fixes a bug with Heroku.
\#4 make before_filter authentication automatic and restricted to :create

* refactor and separate rails/non-rails concerns between Mandrill::Rails::WebHookProcessor and Mandrill::WebHook::Processor