Remove DIY cache system

This branch removes osmdroid's caching mechanisms in favor of using
OkHttp's cache. This means we don't have to maintain a cache, and it
also means that we can cache things transparently. The osmdroid cache
also was 2 step:

http request -> file
file -> map

I don't think this could be very good for performance. Right now
that 2-step is not removed yet, so no tiles appear on this branch.
That is the task to do.
Remove DIY cache system

This branch removes osmdroid's caching mechanisms in favor of using
OkHttp's cache. This means we don't have to maintain a cache, and it
also means that we can cache things transparently. The osmdroid cache
also was 2 step:

http request -> file
file -> map

I don't think this could be very good for performance. Right now
that 2-step is not removed yet, so no tiles appear on this branch.
That is the task to do.
Merge
First tile-serving version of cachebash

* Don't return images to pool so images appear
* American spellings
* simplify map cache implementations
* Remove menu code in tile layers
* Centralize hdpi code
* Remove unused imports
Merge pull request #178 from mapbox/cachebash

Remove DIY cache system