Add support for SSL contexts

[dlocke-np]

I'd like to create an HTTPS connection to an internal server that's using a self-signed cert. I'd like to pass a SSL context to the underlying invocation of httplib at https://github.com/kiorky/SOAPpy/blob/develop/src/SOAPpy/Client.py#L208 but currently that call is still using the deprecated key_file and cert_file arguments.