Recipient Provider implementation

Initial implementation of the recipient provider extension point.
Fix JENKINS-21912

Added help file for the TRIGGER_NAME token.
Attachments test.
Support managed files.
Remove deprecated usage of StreamListener constructor
Retain backwards compatibility
Remove dependency on the Maven plugin
Mode updates...
More backwards compat stuff
Rename class (fix misspelling)
Cleanup
Cache the ConfigProvider.
Update help inline with JENKINS-20384
Mode changes
Merge branch 'recipient_provider'
Add logger to Groovy and Jelly engine bindings.
Add clearing of mailbox between tests.