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.
Allow the logging configuration file to be called logback.xml which is one of the standard names. By default logback looks for files named logback.groovy, logback-test.xml, and logback.xml. Using the standard name allows it to be used much more easily with other tools like Eclipse and also makes it obvious that logback is being used so that it is easier to lookup reference materials for logging configuration
Improve Java Json documentation
Merge pull request #3165 from richdougherty/server-split

Split Netty server code into separate subproject
Merge pull request #3166 from benmccann/logback-xml

Allow the logging configuration file to be called logback.xml
Merge pull request #3143 from gmethvin/java-json-docs

Improve Java Json documentation