Improve phi in AccrualFailureDetector, see #2066

* Implementation of phi according to the paper
* Config properties and documentation, min-std-deviation,
* acceptable-lost-heartbeats
* Restructure code, HeartbeatHistory is responsible for
  stats from historical heartbeats
* Correct and efficient calculation of mean and standard
  deviation
* More tests

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

@@ -16,9 +16,11 @@ class ClusterConfigSpec extends AkkaSpec {
     "be able to parse generic cluster config elements" in {
       val settings = new ClusterSettings(system.settings.config, system.name)
       import settings._
-      FailureDetectorThreshold must be(8)
+      FailureDetectorThreshold must be(8.0 plusOrMinus 0.0001)
       FailureDetectorMaxSampleSize must be(1000)
       FailureDetectorImplementationClass must be(None)
+      FailureDetectorMinStdDeviation must be(100 millis)
+      FailureDetectorAcceptableLostHeartbeats must be(3.0 plusOrMinus 0.0001)
       NodeToJoin must be(None)
       PeriodicTasksInitialDelay must be(1 seconds)
       GossipInterval must be(1 second)