Add Papertrail hook

While Papertrail recommends using their remote syslog adapter, some hosting environments do not allow for that. This hook writes log messages to Papertrail's UDP interface.

Please note that the included test spins up a local UDP server. If this is too brittle or intrusive I can look into mocking that out.

Thanks for Logrus!
The number of bytes written would be useful debugging information (first value returned from `Write`).