(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