add optional jarPath setting - complete path to .jar file

if present, it should be a complete path (including filename) to the jar file
if not, the original closurePath + '/build/compiler.jar' is used

example (needed in debian):
		'closure-compiler': {
			frontend: {
				closurePath: '/usr/bin/closure-compiler',
				jarPath: '/usr/share/java/closure-compiler.jar',
				js: 'js/*.js',
				...
			}
		}

it is an (IMHO simpler) alternative to pull request #36