UHD based demod script

Hi, I've modified the usrp2 script to use the UHD gnuradio driver for new USRP (UHD-only) hardware. It seems to work with my N200+WBX setup, but gnuradio is pretty new to me, so there might be some bugs. Since I'm using the WBX my antenna port is 'TX/RX'. I'm not sure if this is a good default value, it's configurable though. Clemens Hopfer