Update documentation for App Engine deployments.

Hi, this is a patch for the docs, to steer people away from the gae server adapter since it is only useful for old app engine deployments. The Python 2.7 runtime environment supports WSGI applications directly and since Jan 2014 you can't create a new Google app engine application with the  2.5 runtime anyway.

I'm more than happy to work on the wording in this patch if you think it needs it.