Implement the full /stats http api call

Implements the full `/stats` http api call.

```erlang
C:get_server_stats().
{ok,[{<<"riak_kv_stat_ts">>,1377199922},
     {<<"vnode_gets">>,0},
     {<<"vnode_gets_total">>,230},
     {<<"vnode_puts">>,0},
     {<<"vnode_puts_total">>,27},
     {<<"vnode_index_reads">>,0},
     {<<"vnode_index_reads_total">>,6116},
     {<<"vnode_index_writes">>,0},
     {<<"vnode_index_writes_total">>,24},
     {<<"vnode_index_writes_postings">>,0},
     {<<"vnode_index_writes_postings_total">>,0},
     {<<"vnode_index_deletes">>,0},
     {<<"vnode_index_deletes_total">>,0},
     {<<"vnode_index_deletes_postings">>,0},
     {<<"vnode_index_deletes_postings_total">>,0},
     {<<"node_gets">>,0},
     {<<"node_gets_total">>,77},
     {<<"node_get_fsm_siblings_mean">>,0},
     {<<"node_get_fsm_siblings_median">>,0},
     {<<"node_get_fsm_siblings_95">>,0},
     {<<"node_get_fsm_sibling"...>>,0},
     {<<"node_get_fsm_sib"...>>,0},
     {<<"node_get_fsm"...>>,0},
     {<<"node_get"...>>,0},
     {<<"node"...>>,0},
     {<<...>>,...},
     {...}|...]}
```