Incorporated sbt-web concerns into Play in order to reduce the repetition for sbt-web plugin configuration.

The documentation has been updated to reflect sbt-web usage. In addition some references to the now, defunct Play command have also been removed.

Finally the play start command has been changed so that it is now a convenience function for invoking stage and its associated binary. This was done because the sbt-web asset pipeline must run. However I think that start is now more reliable as a consequence.