Make the lightbox work with image-deferrer (bug 1030159)

https://bugzilla.mozilla.org/show_bug.cgi?id=1030159

Losing the swipe w/ snap made me sad, so here is my attempt to keep flipsnap, making image deferring work with it.

TODO:
- [ ] Rebase after feed changes
- [ ] Rebase and convert tests after casper changes
- [ ] Investigate and fix issues with flipsnap and privileged packaged app (Maybe I can do that by forcing a more restricted CSP policy locally, that way I can debug without having to generate a packaged app everytime)