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
Created top level manager for TCP based IO #2887
update/improve design docs as background info for IO layer, see #2890
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
Merge remote-tracking branch 'origin/wip-IO' into wip-IO-TcpConnectionActors

Conflicts:
	akka-io/src/main/scala/akka/io/Tcp.scala
replace `NAck` with the more generic `CommandFailed`
use common naming `commander` instead of `handler`
make incoming connections are set to non-blocking
properly anchor the direct buffer config inside the tcp extension
support for SocketOptions
fix typo in error message