Add Capybara 2.0 support

See: #136 Capybara 2.0 support

Things changed:

* Server#url is no more. It's a Session responsibility and happens transparently from the Driver's point of view.
* Session is responsible for starting the server
* Use Capybara::SpecHelper.run_specs to run validate all specs with the driver
* render is now save_screenshot  (for a failing spec)
* set_cookie needed a domain in specs eg: ```:domain => '127.0.0.1'```  (for a failing spec)
* Normalized whitespace stripping much like capybara-webkit is doing (for a failing spec)

All but one capybara's spec are passing: 

* HTML5 Multiple File Upload: This is from the capybara specs and also didn't found proper documentation in phantomjs that would lead me into having this fixed...

@jonleighton I'm calling for help here to fix multiple file uploads