Rewrites Mail_Parse::parsePriority function

Replaced previous pull request https://github.com/osTicket/osTicket-1.8/pull/348
Includes best function as tested/defined. 4526e54

Includes Test class as described by greezy https://github.com/osTicket/osTicket-1.8/pull/348#issuecomment-31326865 cb316f6

Tested repeatedly with old function (failed miserably), and new function, passed beautifully. Test designed to determine best function, modified to ensure function operates as intended, original test https://gist.github.com/clonemeagain/8066113
Future tests should therefore pass once entire pull request is merged. I'm liking the idea of adding tests!

Also, includes modification to test suite allowing individual test suite to run, instead of forcing ALL tests to run sequentially. Only useful for testing tests.. but still useful. 4822b86

Original issue: https://github.com/osTicket/osTicket-1.8/issues/343