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)
This is meant to accomplish the transition between placeholder and loaded bmp into the ImageView. I'm not really happy with this solution, because the transition sometimes is choppy (the drawable does not sync with the animation framework) and also I have no enough understanding of the management of the state of this drawable (does it hold the state of the children?)