SLING-4240 SlightlyScriptEngine improvements

* Share ResourceResolver for locating script resources with the script
   execution environment instead of recreating each time
* Improve compiler error reporting
* Improvide discovery of script changes to limit recompilation and
   leverage caching of loaded classes in the DynamicClassloader
   
(applied slightly modified patch by Radu Cotescu; thanks a lot)

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1645651 13f79535-47bb-0310-9956-ffa450edef68