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 class should be nuked as well. Even tho is good to have an helper class to help with actually load the bmp into the ImageView this is probably the wrong approach: if you start to use custom components can be really hard to deal with all the different scenarios. The solution is to move this (along with the callback) out from the static one-fit-all setup and let the user to specify one `ImageSetter` (or having a good default)