Upnp ext add NOTIFY SUBSCRIBE UNSUBSCRIBE methods

NOTIFY / SUBSCRIBE / UNSUBSCRIBE are defined by UPnP
See UPnP device Architecture v1.1 document :
http://upnp.org/specs/arch/UPnP-arch-DeviceArchitecture-v1.1.pdf

It is used here : https://github.com/miniupnp/miniupnp/tree/master/miniupnpc-libevent