Updates to mailer and subscription classes to improve testability
- wrapper for mail() to allow overriding in test mock classes
- mailer property and accessor in subscription to allow alternate
  mailer objects/classes in testing
Add tests for Mailer::send()
more Mailer() tests for unusual/edge cases
Improvements to make Mailer() code more robust
don't reuse Mailer object
Merge pull request #462 from splitbrain/mail_headers_ex

Updates to mailer and subscription classes to improve testability