Split Netty server code into separate subproject

This change splits out Play's generic server code and Netty server code
into new projects. Later, I'm going to add a new akka-http server
project, so it's nice to get the Netty code out of the core Play
module.

Before this change, the "play" project included server code and
had a dependency on Netty.

    netty <- play

Now that I've split out the generic and Netty server code, the
play project no longer has a dependency on Netty. I introduced
several new projects:

                                      netty <-.
    play-netty-utils <- play <- play-server <- play-netty-server

* play-netty-utils. Contains cookie encoding and decoding logic
  that I pulled out of Netty's 3.9 branch. The files have been
  added under the play.core.netty.utils package. (Netty has the
  same license as Play and I preserved the source file headers.)

* play-server. Generic server code. There's not much here yet.

* play-netty-server. Netty server code. All the bits we need to
  launch a Netty server.

Existing Play applications will now depend on play-netty-server
rather than on play. I've updated the Play plugin to automatically
add this new dependency into projects.

I've tried to minimise the changes to Play's code. The main thing
I needed to do was add a server-agnostic representation for
WebSocket frames and formatting, but this wasn't an API change for
users. The other thing I did was move some play.core classes into
different packages.