Improve documentation.
Added content length to get the package to work again.
Merge pull request #11 from aiten/master

This fixes the Content-Length issue
Increase revision thanks to content-length fix.
Merge pull request #5 from ghost/master

minor documentation change
Revert proposed fix.
Version 0.1.8 fixes recent content-length change.
Merge pull request #7 from mblackshaw/master

Including the Postmark ErrorCode as part of the error callback