Allow specifying a default reservoir for a registry.

Motivation:

 A usage pattern for the metrics library is to create a MetricRegistry
 and use the counter(), histogram(), meter() and timer() methods to
 create metrics for the application.  When these methods are called
 MetricRegistry potentially creates new metrics with their default
 parameters.

 Creating metrics with non-default parameters, in particular a
 non-default reservoir, currently requires explicitly creating the
 metric and then registering it and remembering it for future use.
 This prevents leveraging functionality already provided by
 MetricsRegistry.

Proposed approach:
 Allow specifying a ReservoirBuilder at MetricsRegistry creation
 time.  ReservoirBuilder creates new reservoirs with custom
 parameters.  This allows the creation of histograms and timers
 with custom default reservoirs.

Alternate approaches:
 A potentially more comprehensive approach would be to allow
 passing a custom "MetricsBuilder" to the MetricsRegistry
 constructor.  This approach is definitely worth exploring.

Closes #679