Add disk stats to `/stats` output to make consistent

As pointed out by @fygrave in #285, `riak-admin status` shows
diak stats, but `/stats` does not. This fixes that.#285, `riak-admin status` shows
diak stats, but `/stats` does not. This fixes that.#285, `riak-admin status` shows
diak stats, but `/stats` does not. This fixes that.
Merge pull request #383 from basho/rdb-eq-stats-status

Add disk stats to `/stats` output to make consistent