Update class.mailparse.php

Best version of https://github.com/osTicket/osTicket-1.8/pull/348

static keyword added to enable strict test mode.
Update run-tests.php

Allow tester to run single test by specifying it on command line: php run-tests.php TestMyThing etc.

Still works for normal tests.
Create test.header_functions.php

Creates a regression test for the parsePriority function, as suggested by GreezyBacon https://github.com/osTicket/osTicket-1.8/pull/348#issuecomment-31326865
Merge pull request #491 from clonemeagain/#348

Rewrites Mail_Parse::parsePriority function

Reviewed-By: Peter & Jared