Imageview

Refs #241 : this work removes much of the existing codebase to see if we can rewrite it smaller and better. Thoughts are:

* Use ImageView for tiles
* Remove tile downloading cruft, replace with picasso and built-in caches
* Remove Overlay abstractions entirely, create `ILayer` with a simple scheme, like `.draw()`
* Remove separation between Overlays & TileLayers, map should have a single `ArrayList<ILayer>` and call `.draw()` on each every time the map needs to update