Load images as resources when 'img' feature enabled

This patch will load the actual image content (as a buffer) of 'img'-tags into resources, but only if the 'img'-feature is enabled.

document.coffee: Adds feature 'img'
jsdom_patches.coffee: Adds jsdom patch to allow actually loading img tags (instead of only triggering the 'load' event).
resources.coffee: Adds exception for decoding response body to string for images: those will stay as buffers, like octet-streams.