First stab at issue #49: Unsupported major.minor version 51.0 with JRE6.
Merge pull request #50 from abirmingham/master

Added sourceCompatibility/targetCompatibility for Java 6
Allowing options and output to stdout

- Also defaulting dest to a file, so that it is not required (e.g. when directing output to stdout)

Conflicts:
	src/main/groovy/com/eriwen/gradle/js/JsPlugin.groovy