postmark 6.x-1.0

Fixes relating to most of the issues in the issue queue.

Also updated to reflect new release of the Postmark PHP class, please ensure you update to the new version 0.4.4, see README.txt for more details.

#906838: Sending to multiple 'To' addresses sends only to the final address
#925404: Update README.txt for Postmark module
#877592: Postmark module seems to remove line endings
#925582: better handling for exceptions
#967638: Does this module use the REST API or the SMTP endpoint?
#1004340: When postmark is disabled, drupal still tries to send email through postmark