Reintroduce the 'DocumentWrapper' for document compression

- basically a revert of 060cd0b354137a954e9fbc627c8e853511048baa
- resolves #16
Add support for document compression with google's snappy library

- Document.Compression split into
  - Document.Compression.BZip
  - Document.Compression.Snappy
Refactor occurrence compression and add google's snappy
Use snappy instead of bzip as index compression
Cleanup
Remove unnecessary ByteString newtype in Occurrences.Compression