Add secure XML-RPC

The idea is to build a secure XML-RPC messaging feature on top of what's available in the stdlib's modules, and probably with some bits using PyOpenSSL.

By secure I mean:

• clients should be able to use SSL certificates
• servers should be able to use SSL certificates
• both clients and servers should be able to validate whether certificates have been signed by a given CA or a chain of CAs
• servers should be able to further validate client's certificate on the most common fields, such commonName, so that's possible to say that a certificate not only must be signed by the FooBar CA but also that it's commonName must be equal to BazFroGor