[WIP] Import cluster implementation by @bsm

This is a WIP to bring @bsm's cluster implementation to Sarama. 

TODO / discussion:

- Do we want to keep it in a separate package or merge it into the main sarama package?
- Tests need to be rewritten to fit the rest of the library.
- Let's use a standard UUID instead of the custom guid implementation.

/cc @eapache @burke