Add pxfetcher modules without deleting existing imgloader code

As requested by @mr-archano @blundell @xrigau, a less diff-ful version of the previous PR (aka, the original imageloader modules still exist, I just changed a bit to make it compile with this version of gradle android plugin).

This adds the code we used in our latest project - a wrapper of sorts around the original imageloader. There are four modules now:

- core (as before)
- demo (as before, still works)
- pxfetcher (the new core library, has dependency on core)
- pxfetcher-demo (the new demo, has dependency on pxfetcher)
The comments for the `ImageSetter` apply pretty much here too. It should be simple to change the callback, so this should not be part of the core setup, but something the client should deal with (and yes, avoiding that later with nice defaults in the final API)