Capture and expose mandrill return value for submitted email.
This allows you to access the result through the `mail#delivery_method`
which is an instance of MandrillActionMailer.

 * Add `result` attr_accessor on MandrillActionMailer
 * Populate `result` with the return json from mandrill
   which includes id, status, reject_reason, etc.
 * Rename integration test to match handler name
 * Set values for track_opens and track_clicks in env
   and add test to ensure those are set in mailer.