Add Search Caching

[aag]

The current code does a YouTube search for every song in every playlist. This is a problem because the current Google API credits system ranks searches as very expensive, so a normal account doesn't have enough credits to search for all the songs in every playlist in a single day.

Since the songs in the charts don't change that quickly, and some songs are in multiple playlists, we can do better by caching the search results. We should write the video ID and creation date to a file (perhaps a Shelve or an SQLite file) each time we do a search, and always look for an entry in the cache before doing a search. We could also clean up old entries after a certain amount of time, maybe a year.