Support REST API as backend.

Allow to use a REST API server as backend for NMControl.  Supports both HTTP and HTTPS connections, and allows to optionally specify the expected TLS certificate.  This makes it safe to use with a remote host.

Note, though, that I'm not a TLS nor Python expert.  So I can't guarantee that the code is safe (but I expect it to be, review welcome).

See https://github.com/namecoin/nmcontrol/issues/48.