Created top level manager for TCP based IO #2887
Merge pull request #1019 from spray/wip-IO

update/improve design docs as background info for IO layer, see #2890
Merge pull request #1017 from drewhk/wip-2887-tcpmanager-drewhk

Created top level manager for TCP based IO #2887
tcp connection actors, see #2886
initial versions of tcp selector and listener actors, extension of tcp manager actor, see #2885 and #2887