[EM] Added adapter for Cloudmailin email service.
Fixed a typo in the test and fixed the code to make it pass. Cloudmailin doesn't pass attachments at all if there aren't any so I decided it should be an empty array in this case.
Merge pull request #37 from r38y/cloudmailin-typo

Fixed a typo in the test and fixed the code to make it pass. Cloudmailin...
[#33] Parsed headers support

Uses Mail gem to parse optional headers in the params hash.