GRADLE-902 Adds ANTLR v3 support to existing antlr plugin.

My solution for this issue: http://issues.gradle.org/browse/GRADLE-902

There appeared to be some debate as to whether this functionality should be included in a new plugin or the existing one.  This implementation adds it to the existing antlr plugin by checking for a property ('antlrVersion') on the generateGrammarSource task.  If not found, it will default to ANTLR v2 for backward compatibility.