Incorparate review feedback, see #2547

* case object and case class for MixMetricsSelector
* Rename decay-half-life-duration to moving-average-half-life
* Clarification of decay-half-life-duration and collect-interval
* Removed Fields, Java compatibility issue
* Adapt for-yield variables
* Comment metrics collector constructor that takes system param
* Don't copy EWMA if not needed
* LogOf2 constant 0.69315
* Don't use mapValues
* Remove RichInt conversion
* sigar version replace tag in docs
* createDeployer factory method to make it possible to override
  deployer in subclass
* Improve readability of MetricsListener (in sample)
* Better startup of factorial sample (no sleep)
* Many minor enhancements and cleanups

akka-cluster/src/test/scala/akka/cluster/ClusterConfigSpec.scala

@@ -50,7 +50,7 @@ class ClusterConfigSpec extends AkkaSpec {
       MetricsCollectorClass must be(classOf[SigarMetricsCollector].getName)
       MetricsInterval must be(3 seconds)
       MetricsGossipInterval must be(3 seconds)
-      MetricsDecayHalfLifeDuration must be(12 seconds)
+      MetricsMovingAverageHalfLife must be(12 seconds)
     }
   }
 }