Added appengine support

[brutuscat]

I added support for Appengine implementing a Transport class for xmlrpclib that uses urlfetch instead of sockets. The knowledge for doing it came from this article: http://brizzled.clapper.org/id/80/

[maxcutler]

In version 2.3 I've exposed a transport parameter that you can pass with your custom transports. I'm not sure it makes sense to bundle particular transports in the library, but you should feel free to put it in a separate package and upload it to PyPI for easy sharing and consumption.