Cleanup UPnP API

This commit addresses most of the issues identified in #432:

* Support UPnP IGDs with both WANIPConnection and WANPPPConnection services

  IGDs that offer both WANIPConnection and WANPPPConnection services should
  now have port forwarding correctly configured for all services.

* Support multiple UPnP WANDevice and WANConnection descriptions

  Per Figure 1 of the InternetGatewayDevice specification
  (http://upnp.org/specs/gw/UPnP-gw-InternetGatewayDevice-v1-Device.pdf),
  an IGD may have multiple WAN devices, each with multiple WANConnection
  services

* Support for discovery of UPnP InternetGatewayDevice version 2 devices

* Support for discovery of multiple UPnP IGDs

  Consumers that cannot yet properly process multiple IGDs can simply use
  the first IGD listed in the discovery results

* Logging refinements such as friendly UPnP IGD identifiers in log messages.