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.
Added test for image resources/tags
Merge branch 'master' into pr/575

Conflicts:
	src/zombie/jsdom_patches.coffee