Add host() and port() to mobileproxy

[fortuna]

Currently mobileproxy.RunProxy returns a mobileproxy.Proxy with a address getter, which returns a host:port string.
However, most code to configure the network stack needs separate host string and port number. This means the client has to split the string, and convert the port to number.

To make it easier on the client, we should provide Proxy.host() and Proxy.port() instead.
Or perhaps return an Address type with those methods.

[fortuna]

Implemented in #107