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.

Depends on basho/riak_ee#295
I think it's just my general misunderstanding of how status types ought to be used, but easily fixed.