call_prop with nested properties

ExecJS's `call` supports nested property access. Which lets you do `call("CoffeScript.compile", data)`.

At the moment, `call_prop` only supports calling global properties. Theres no way to get a reference to any nested property which makes it very limited.

@judofyr Do you think this is something we should support?

My idea for the implementation would be eval and get the object reference for the property, then use `duk_call`.
Whats the preferred convention for iterating over a Ruby Array? Whatever I'm doing here doesn't work in 1.9.