Please help review the Frog responsive images support branch

[gerdint]

Modeled after Wordpress' responsive image support I implemented a similiar feature for Frog some time ago. This is pull request 166.

Greg is very busy and while he has partially reviewed the code he suggested I asked other Frog users for impressions and code review. I believe this is an important feature given today's very diverse client landscape (in particular for an image-intensive blog like my own).