Caching for online audio assets.

[GetToSet]

Currently Doughnut has no caching behavior for online assets. A carefully implemented caching could improve the following aspects:

1. smoother seeking on a poor network environment;
2. allow playback to be continued across restart, even when offline, without explicit downloading;
3. reduce memory usage;

DVAssetLoaderDelegate, VIMediaCache or CachingPlayerItem would probably be a starting point.