Rebase of previous pull to execute spec tests and integrate ibrahima comment:

* Allow @message @source in match for email output
* Restore lost plugin documentation from some time ago
  https://github.com/logstash/logstash/commit/6c9b36b2279643711b0e5e41a366b0940fac0068
* Integrate ibrahima comments: use respond_to? :each
 Update CHANGELOG
Merge pull request #480 from wiibaa/output-mails

Allow @message, @source etc in match for email output