Added optional support for custom_request_handler_class to AMs

[wvdemeer]

These changes allow an AM implementation to specify a custom request_handler_class to SecureXMLRPCServer. (By adding a static class variable custom_request_handler_class to the class specified as delegate in the config.)

I've used this to add a non XML-RPC site and web service to the AM (both using the same client SSL authentication as the AM).

The small changes in this pull request are quite generic and don't change anything if you don't add custom_request_handler_class to the delegate. So it might be useful to add them.

[dmargery]

I would certainly use this to host xml schema definitions that are specific to the delegate

[ahelsing]

Looks OK to me from eyeballing it. Users will have to be careful as I'm sure various places in the code assume that the request handler has grabbed the peer certificate, but that's up to the developer.