Refactor server.py and handle missing README.
Cache style assets:

* Move cache to cache/ instead of 'style-cache'
* Rename STYLE_CACHE_DIRECTORY to CACHE_DIRECTORY
* Make the 'cache' endpoint configurable with CACHE_URL
* Find and download all assets from the downloaded styles
* Do a regex replace on the styles so they point to the correct local assets
* Handle IO exceptions with cache files and their directories
Move TODO out of code.