Support for column families

[grove]

This would be very nice to have in pyrocksdb.

https://github.com/facebook/rocksdb/wiki/Column-Families

[Downchuck]

Any chance you could start posting some documentation notes in here? Adding the interface looks reasonably easy -- it seems like documenting it is more work.

[grove]

I have a preliminary working version of column family support here for those interested: https://github.com/grove/pyrocksdb/tree/column-families

I'll just have to write tests and documentation before creating a PR. Any feedback on the API mapping would be nice.