Fix for #56 (stop re-caching files that came from cache)

This fixes #56 by checking whether valid responses came from cache or and HTTP request and only writing the HTTP responses to cache. Also this includes a fix to the double-size-prefix bug in the marker url which was introduced by https://github.com/mapbox/mbxmapkit/commit/0affd03869e6ba5aef996743a2c5ff49216ba861 (`if...if...else...` should have been `if...else if...else...`).