Added `riak-admin cluster members`

This lets users see current cluster membership. It does exactly the
same as `riak-admin member-status` only it's in the cluster subcommand,
where I feel it should live.

I kept tripping up on the fact that the cluster membership commands weren't in the same place as the cluster altering commands, so I decided to make it so! Open Source ftw!

The old commands still work, and perhaps there's a better way of keeping backwards compatibility than duplicating code. I guess one can just call the other.