improvements from the first round of feedback, see #2885 and #2887
more tests, smaller improvements
Merge branch 'spray-wip-IO' into wip-IO-TcpConnectionActors

Conflicts:
	akka-io/src/main/scala/akka/io/DirectByteBufferPool.scala
	akka-io/src/main/scala/akka/io/TcpConnection.scala
dispatch Closed events to the sender of the close command as well