Allow enableAutoDownload to be set when using @Grab

The only way to set enableAutoDownload is to run the script with:

groovy -Dgroovy.grape.autoDownload=false SomeScript.groovy

It would be useful to add this to @GrapeConfig or use something like that to allow this to be contained to the script.

As a related thing - I tried using method calls as per http://stackoverflow.com/questions/3632662/disable-automatic-download-for-groovy-grapes. I could not set this to use system class loader to replace import with Class.forName - not sure how this works from within groovy/grape/GrabAnnotationTransformation.java.