Tweak the behaviour of `$.fn.resizeImages`.

- Expose the resize host to allow it to be changed for testing.
- Return only images that were modified.
Merge remote-tracking branch 'origin/master' into tweak-resizeImages
Merge pull request #83 from mobify/tweak-resizeImages

Tweak the behaviour of `$.fn.resizeImages`.
Merge remote-tracking branch 'origin/master' into image-cache-expiry