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.
Just my personal style opinion, but I don't like the brackets on separate lines in erlang. I prefer something like this, but It's your decision.
```erlang 
       [["riak-admin", "cluster", "status"], % Cmd
       [],                                  % KeySpecs
       [],                                  % FlagSpecs
      fun status/2].                % Implementation callback
```