JMS input plugin addition

This plugin adds the possibility to consume messages from a JMS provider.
It solves the feature request https://logstash.jira.com/browse/LOGSTASH-1862

This plugin adds a dependency to the jruby-jms gem in the contrib tarball (https://github.com/reidmorrison/jruby-jms).
The jruby-jms gem seems to be very stable, and has already been tested with many JMS brokers.
But, this plugin has been developed and tested with an Oracle AQ provider only.
So, additionnal tests are necessary with other JMS providers like ActiveMQ, HornetQ, IBM WebSphere...

Code review and feedback from Logstash developpers and designers is appreciated, and necessary ! (see TODO notes in jms.rb)

new file:   lib/logstash/inputs/jms.rb
modified:   logstash-contrib.gemspec