Extracting the actual address the host was bound to

Typical use case: you're creating a host, you don't care about the port number so you're just using `ENET_PORT_ANY`, but after the host is created you'd like to show the user on what port the server actually listens.

If you'd check newly created `ENetHost`'s `address.port`, you'd find `ENET_PORT_ANY`. Now you'll find the correct, actual port the host is bound to.

This also exposes underlaying function (`enet_socket_get_address()`) as a part of API.

I have not tested this on Win32 (had no means to do so). On both Linux and OS X it works well, and the change itself is quite trivial, though it definitely should be tested on Win32 too.

Oh, and I guess this change is not backwards compatible.