Allow to get a property of a remote object

This allow to get a property of a remote object.
Usage: Casper.getGlobal('foo[0].bar.baz');
(in my use case, I can not use Casper.getGlobal('foo')[0].bar.baz because foo contains circular references and JSON.stringify fails).

I presume this patch is not a very good idea, too complex, and I should just use evaluate to get the property I need.