callback on non 200 status

fix end event never being emitted and callback not being triggered if any resource returned a non 200 status.

For example  when running inliner against http://remysharp.com the progress events show that three resources return non 200 status codes:
get 301 on http://twitter.com/api/users/profile_image/rem
get 301 on http://remy.github.com/twitterlib/twitterlib.min.js
get 404 on http://leftlogic.com/images/favicon.png

This updates Inliner.get() to call its callback with and empty string if the resource returns a non 200 response.