Milestone Search server: Xapian search engine

In this milestone the Lucene search engine will be replaced with the Xapian search engine. This will allows easier installation of search servers (this has been a BIG deal in the past), gives better search results, builds indexes faster, allows stop words in artist searches, and will hopefully allow us to handle more searches per single server.

We're now handling about 1,500 searches per minute, so search has become quite important for us.