Towards Metrics 3

I've begun work towards support for Metrics 3. The specs are now passing (but I've not yet run the code).

Some things to note:

* the import of `JmxReporter` has been commented out temporarily (specs still pass). It needs to be updated.
* Options when registering metrics have been dropped. `biased`, `duration_unit`, `time_unit`, `event_type`, `rate_unit`. The ruby method signatures should likely be updated and/or raise an error if these options are given. Or equivalent functionality should be added.
* Histogram and Timer will no longer return a `sum` -- since that method can no longer be found as far as I can tell. It could potentially be calculated though.