Extend service types

This feature branch adds the WANPPPConnection service type, which as I understand it now is exactly the same as WANIPConnection as far as Mono.NAT is concerned (Add/Delete port maps).

I removed InternetGatewayDevice from the device capture list as its really only a container for the above service types that contain the actual port mapping methods. The IGD service is good for grabbing lots of information about the device itself, but the way Mono.NAT works right now doesn't really support collecting these services into a single cohesive 'Device' object unfortunately.
Maybe in future we could think about refactoring it a bit to be more supportive to collections of services under a single device object.