Allow converting Stapfen::Destination objects to JMS compatible qualified strings
Introduce a Stapfen::Client abstraction to unify the worker API code betwen STOMP and JMS

This requires more testing but thus far I've done some integration testing with an internal project
Add a few more specs for Stapfen::Client::JMS

Testing more parts of the #publish method might need to be left to integration
tests, otherwise we'll probably be stubbing out far too much of the "meat" of
the method