Merge pull request #1009 from spray/wip-IO

Add first version of TCP message protocol
add design docs as background info for IO layer, see #2890
Merge pull request #1010 from akka/wip-2890-design-docs-∂π

add design docs as background info for IO layer, see #2890
add Tcp IO extension and first draft of settings/options, see #2888
Merge pull request #1014 from akka/wip-2888-IO-extension-∂π

add Tcp IO extension and first draft of settings/options, see #2888
update/improve design docs as background info for IO layer, see #2890