WeMo plugin added

@mzanetti I just finished the WeMo plugin. I tested many possible situations (discover, add, discover and add when allready added, switch on/off, switch on/off when allready on/off....). I think all situations are covered and work great. 

The documentation is still missing, I will do that as soon as possible.
you leak all previously discovered devices...

However, if you delete them here you might get problems too because you emit them in a signal... I'd suggest you change the discovery to create and emit a lit of DeviceDescripts (not pointers). You can then just pass them on to gu and not worry about deleting them. Then you create the actual WemoSwitch object in setupDevice() and maintain that object in the plugin only.