Add stats formatting for riak-admin and http stats output

YZ stats are not currently displayed on the riak http stats endpoint, or by riak-admin status. Add YZ stats formatting appropriate for console output with 'riak-admin status' and the htts /stats endpoint. The stats namespace for Search is transformed to /search/[query | index]/... instead of 'yokozuna'.

This PR also depends on changes to riak_kv in order to publish the stats to the 2 interfaces:

https://github.com/basho/riak_kv/pull/855

Example stats output:

search_query_fail_count : 0
search_query_fail_one : 0
search_index_throughput_count : 0
search_index_throughput_one : 0
search_index_fail_count : 0
search_index_fail_one : 0
search_query_throughput_count : 0
search_query_throughput_one : 0
search_query_latency_min : 0.0
search_query_latency_max : 0.0
search_query_latency_median : 0.0
search_query_latency_percentile_95 : 0.0
search_query_latency_percentile_99 : 0.0
search_index_latency_min : 0.0
search_index_latency_max : 0.0
search_index_latency_median : 0.0
search_index_latency_percentile_95 : 0.0
search_index_latency_percentile_99 : 0.0