Add support for HTTP SUBSCRIBE verb

[mcclurec+flic]

I'm trying to implement volume support for Wiim devices via a Flic Twist. Wiims present as a generic UPnP media renderer, so sending them volume change commands is pretty trivial from the Twist. What I want to do is get the volume states back from those devices if another application changes their volume. UPnP uses the SUBSCRIBE HTTP verb with a supplied callback endpoint, which in this case is just the Flic Hub IP address. It looks like you guys restrict the specific verbs which we're allowed to specify. If you're able to allow this new verb, we can make a generic app that supports all UPnP media renderers for native volume control.

[Emil]

@mcclurec-flic Right... In the meantime, you can use raw TCP sockets to send the request.