Make reservoir configurable

[venikkin]

Reservoir used in internal meters (e.g. ThroughputMeter) is hardcoded to the default ExponentiallyDecayingReservoir. Although it's possible to provide MetricRegistry implementation working with any other reservoir, internal meters still will be using default one.
I suggest a PR #82 allowing specifying reservoir factory. Added only java way to set the factory so far.

Appreciate your feedback.
Thanks

[tsegismont]

I had a look at your PR. If we make the reservoir type configurable, I believe it should be per-metric.