Add *args option to execute_script

This PR aims to do nearly the same as https://github.com/jnicklas/capybara/pull/564

However, this one:
* adds tests
* casts args of type `Capybara::Node::Element` to native
* casts returned value to `Capybara::Driver::Node` (I don't know how to create proper `Capybara::Node::Element` from driver.rb)

Parameters [are supported by PhantomJS](https://github.com/ariya/phantomjs/pull/231) so it will be possible to add this functionality to at least Poltergeist.

I made it a PR as it changes public API.