(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.
(PE-2650) update tests/examples

This patch updates the examples and tests contained in this project
for the new trapperkeeper defservice syntax and also makes sure
that the example rack applications are on the classpath so that they
can be found by the add-rack-handler function provided by this project.