Allow plugin to be used as a "must-use" plugin

[atheken]

See here for an explanation of the feature: http://gregrickaby.com/create-mu-plugin-for-wordpress/

[mgibbs189]

Maybe this is worth its own repo?

The code to convert this into a MU plugin wouldn't be all that different... it'd just involve stripping out the UI and supporting the necessary constants:

define( 'POSTMARK_API_KEY', '' );
define( 'POSTMARK_SENDER_ADDRESS', '[email protected]' );
define( 'POSTMARK_FORCE_HTML', true );
define( 'POSTMARK_TRACK_OPENS', true );