This code borrows and fixes up a chunk of code to handle upnp from
Taipei-Torrent (https://github.com/jackpal/Taipei-Torrent), under
current versions of go none of the xml parsing was working correctly.
This fixes that and also refactors the SOAP code to be a little nicer by
stripping off the soap containers. It is still rather rough but seems to
correctly redirect ports and advertise the correct address.
Upnp is not run by default. --upnp will enable it, but it will still not
run if we are not listening or if --externalip is in use.
Closes#51
