* MetricsSelector, calculate capacity, weights and allocate weighted
routee refs
* ClusterLoadBalancingRouterSpec
* Optional heap max
* Constants for the metric fields
* Refactoring of Metric and decay
* Rewrite of DataStreamSpec
* Correction of EWMA and removal of BigInt, BigDecimal
* Separation of MetricsCollector into trait and two classes,
SigarMetricsCollector and JmxMetricsCollector
* This will reduce cost when sigar is not installed, such as
avoiding throwing and catching exc for every call
* Improved error handling for loading sigar
* Made MetricsCollector implementation configurable
* Tested with sigar
