Include cityhash module

[FedericoCeratto]

Hello and thanks for developing the library. Would you consider including the cityhash library in your codebase? It would greatly simplify packaging the library for Linux and reducing the dependency fragmentation.

Thank you!

[xzkostyan]

Hi.

I've decided to make separate package for compression considering following points:

1. clickhouse-cityhash is dependency with fixed cityhash algorhythm. So it looks like vendored code, that will never change.
2. Portability: clickhouse-driver was pure python package (without cython code). Building wheels was not so easy in 2017. You was able to use the package on any platform. But without compression.

The second point is no a problem anymore. I suppose we can vendor clickhouse-cityhash since it's useful only for clickhouse-driver.