Merge pull request #7 from MrTravisB/master

Add support for missing message params
mandrill does not add DOCTYPE anymore.
integrate changes from James Cooper@Staffis.

Thanks James!
Reintegrate changes from MrTravisB

- SORRY!
Removing omitempty for TrackOpens and TrackClicks to make sure the values are transferred to mandrill as false and not omitted. This is because mandrill seems to default to true for these values, and if they are omitted there is no way to turn off click tracking.
Merge pull request #8 from hwsoderlund/master

Removing omitempty for TrackOpens and TrackClicks
Put id and rejected reason back in send message response