Global Scope

As of v0.1.8, `postmark_headers` is leaking as a global variable. This was previously addressed in https://github.com/voodootikigod/postmark.js/pull/8 (merged by @voodootikigod), but (I assume inadvertently) reverted in https://github.com/voodootikigod/postmark.js/pull/11 . 

Cheers.