Add support for snappy compression in Bitcask backend

The performance of read intensive applications using the Bitcask backend can be enhanced if the operating system level file cache is used efficiently. In order to allow as much data as possible to be cached, one approach is to compress the data stored.

When using bitcask together with Riak Search and/or Yokozuna, storing compressed data complicates the indexing process, and it would instead be desirable to have the backend handle this internally like LevelDB does, which is why I have added support for snappy compression to Bitcask.