JRubyExec Task

The main feature here is the addition of ```JRubyExec``` task. A number of important changes were made to existing classes as part of the process:

* ```JRubyPlugin.gem*``` static methods have been refactored into ```GemUtils``` class.
* Additional ```GemUtils.extractGem``` emthods has been added to use ```jruby-complete``` instead of installed ruby.
* ```JRubyExtension``` as an ```execVersion``` version added. Setting this will cause all existing ```JRubyExec``` instances to be updated if necessary.
* Spock Framework was added for improved unit testing as Junit4 was to restrictive for certain test cases.
* Certain test cases will be ignored if ```gradle``` is run with ```--offline```.