Allow image loading to be disabled

Hi,

The following patch allows image loading to be disabled in the Webkit driver.

This is done in a similar fashion to how the ignore_ssl_errors option is implemented. The skip_image_loading option is passed down to the webkit_server process which disables auto loading of images on the QWebPage object.

This gives a decent performance boost and bandwidth saving when using capybara-webkit on external websites, as we do in Still Alive.

Cheers,
Sean
Why is this change lumped in?