Add stat/2 callback for retrieving individual stats.
Add tests for stat/2 and stats/1 callbacks.

Also fixed bug in riak_dt_orset stats, which had previously assumed we
were still using {Adds, Removes} per element instead of
[{Tag, IsRemoved}].
Merge pull request #73 from basho/refactor/sdc/stats

Add stat/2 callback for retrieving individual stats.