Merge pull request #574 from FrozenCow/fix_css

Load 'link' tags when loading css is enabled
Load images as resources when 'img' feature enabled

* Adds feature 'img'
* Adds jsdom patch to allow actually loading img tags (instead
  of only triggering the 'load' event).
* Adds exception for decoding response body to string for
  images: those will stay as buffers, like octet-streams.