JavaFX Groovy classpath fix

This PR fixes a problem when compiling Groovy programs using the JavaFX technology from Oracle.
When using the Oracle JVM, Groovy code was being compiled without adding the Extension Classloader to the compile classpath.
The problem is described in [a post](https://groups.google.com/forum/#!topic/gradle-dev/q3u-T09km2s) on the gradle-dev Google group.

Integration tests are included.