Serve Play documentation from a JAR file

This means Play can be distributed without needing to unzip
files onto the filesystem. This will make it easier for Activator
to interact with Play.

* Made new project, play-docs, which includes code for serving
  documentation along with the documentation content.
* The play-docs project takes responsibility for generating the
  api-docs. This happens before every package operation, which
  makes things a bit slow for publishing. That will need to be
  sped up somehow.
* The plugin's run task now runs the documentation application
  in a separate classpath to the user's code.