Porting to new capybara-webkit

1. HEAD of capybara-webkit:
   f35e2df0df3b2e253d78ab96afa670662f862037
2. Porting commands:
   SetAttribute, SetHtml, Wait, Evaluate
3. Remove commands:
   SetViewportSize, SetErrorTolerance, Source
4. New commands:
   IgnoreSslErrors, ResizeWindow, SetSkipImageLoading
5. Extra command to set attached file directly:
   SetAttachedFile