3.0.0: Rework for speed and Rails 4 + Ruby 2.1.

* Support new Ruby 2.0 and 2.1 GC stats.
* Gauge GC details with GC::Profiler.
* Performance rework. Faster, fewer allocations.
* Rework counters and gauges as instruments.
* Batch StatsD messages to decrease overhead on the server.
* Drop NewRelic samplers.