Add support for some extra Riak stats

I'd like to monitor the `node_gets_set`, `node_puts_set` statistics, along with the `node_fsm_get_set_objsize_*` family of stats. This pull request adds both, although the latter in a very hackish way, which should likely be done better.

But my ruby is quite weak, and my time limited, so I wanted to get this out here ASAP. I can reshape the patches - especially the second - later, if so need be.