wake on access - upnp support (v2)

This pr adds support for waking upnp-servers and replaces PR https://github.com/xbmc/xbmc/pull/4104

ref this post and onwards : http://forum.xbmc.org/showthread.php?tid=124340&pid=1611088#pid1611088
and : http://trac.xbmc.org/ticket/14880

Should work fine also for upnp-servers hosted on machines with DHCP assigned ipaddresses since alive-test ('ping') now is done by probing availability of the upnp-server rather than actual ping
I agree in general, and your argument also is valid for the function [ShowDiscoveryMessage() ](https://github.com/t4-ravenbird/xbmc/blob/woa_upnp2/xbmc/network/WakeOnAccess.cpp#L77)
In this case there are 2 reasons for me to do it as I did ; Firstly, the functions are local helpers and not exposed to outside users. Secondly, doing it differently means duplicating a significant chunk of code which is not desirable (unless there is a good way to refactor that I miss?)