Add riak_core stats to riak-admin status

[russelldb]

See #740

[slfritchie]

That's all? Wow, yay! +1

[kuenishi]

This is not directly related, but I found riak_client:get_stats/1 only using riak_kv_stats:get_stats, which is called from riak_snmp . Though I don't know whether it's used or not, +1 from me, too.

[russelldb]

@kuenishi I think snmp/jmx are different issues. Hoping that the exometer work ultimately solves the brittleness around stat export/consumption.