Add stableKey parameter to Downloader.load(...)

This little change gives the user the possibility to implement his own
disc chaching with stableKeys within a custom Downloader.

I down't know if is the best way to do this, but is a way without touching picasso very much. One could achieve the same with a custom Cache, but you wouldn't be able to report correct value for Picasso.LoadedFrom without changing the BitmapHunter.

I don't mind if this does not make it in if you feel this is unnecessary but i think it is a logical step after my last PR wich added StableKeys. This code should have no effect on Picasso core thus the null values in the test classes.