Implement cluster CLI commands

Implement the following commands using riak_cli:

* cluster status (improved member-status)
* partition-count
* partitions
* partition id=\<id\>
* partition index=\<index\>
* members

TODO: After merge, some code clean up and boundary checking.

**NOTE**: This PR depends on a forthcoming PR in riak to support an updated riak-admin. Will link it here when its available.
OK. I don't have a strong preference, but I found it avoided merged conflicts sometimes. I will fix it up.