Mailgun custom headers code was totally wrong. Rewrote it.
Removed stray new lines
let the user decide to remove the html comments on emails
Merge pull request #53 from pendexgabo/1.8/develop

let the user decide to remove the html comments on html body emails
Merge pull request #51 from phil-lavin/bugfix-mailgun-headers

Bugfix mailgun headers
Code cleaning, coding standard, accidentally overwritten copyright info restored
Added Mandrill driver
Added mail priority
Use Arr::keyval_to_assoc for merge var building
CS fix
Dont mess with extra_headers
Throw exception on inline attachments
Make it clear: we only have a problem with non image attachments