Add support for Groovy configuration script
Groovy configuration script support enhancements:
   * nicer error message if version<2.1
   * add @Optional to configurationScript
   * add integration test
   * improved documentation
Groovy configuration script support enhancements:
   * fix integTest assertions
   * improve error message if Groovy on classpath is not 2.1+
   * added integration test for missing config file
   * added integration test for invalid config file
   * added integration test for incremental compilation
Merge pull request #285 from melix/groovy-configscript

Add support for Groovy configuration script