Added ClosureScript class

This is a convenient `Script` subtype that performs method invocations and property access like `Closure` does.

For example, when the script is:

	a = 1;
	b(2);

Using `ClosureScript` as the base class would run it as:

	delegate.a = 1;
	delegate.b(2);

... whereas in plain `Script`, this will be run as:

	binding.setProperty("a",1);
	((Closure)binding.getProperty("b")).call(2);

This makes it a convenient basis for loading a custom-defined DSL as a `Script`, then execute it.
The following sample code illustrates how to do it:

	class MyDSL {
	    public void foo(int x, int y, Closure z) { ... }
	}

	CompilerConfiguration cc = new CompilerConfiguration();
	cc.setScriptBaseClass(ClosureScript.class.getName());
	GroovyShell sh = new GroovyShell(cl,new Binding(),cc);
	ClosureScript script = (ClosureScript)sh.parse(new File("my.dsl"))
	script.setDelegate(new MyDSL());
	script.run();

<tt>my.dsl</tt> can look like this:

	foo(1,2) {
	    ....
	}

I find this useful in a number of places (for example in Jenkins where we define several Groovy DSLs), so I'm trying to push this back to the upstream.