(PE-2650) updates and changes needed for the rack-app-in-a-jar support

Changes introduced in this commit:
 * updates required to make the code compatible with the current
   trapperkeeper (defprotocol + new defservice parameters)
 * the code no longer directly registers the rack application
   with the jetty webserver, instead it uses the add-context-handler
   function recently added to the webserver service for that purpose
 * updates needed so that the code can deal with jar based resources
   equally well as with filesystem based resources
 * dropped dependency on jetty
(PE-2650) rack environment setup improvement

This patch improves the setup of the jruby-rack environment
resulting in less friction when running a rack app from a
jar file.