tcp connection actors, see #2886
tcp selector and listener actors, extension of tcp manager actor, see #2885 and #2887
Merge branch 'spray-wip-IO' into wip-IO-TcpConnectionActors

Conflicts:
	akka-io/src/main/scala/akka/io/Tcp.scala
	akka-io/src/main/scala/akka/io/TcpManager.scala
	akka-io/src/main/scala/akka/io/TcpOutgoingConnection.scala
	akka-io/src/test/scala/akka/io/TcpConnectionSpec.scala