Announcement: GRC USRP2 support and improved USRP stuff

USRP2 source and sinks blocks are now available in GRC. The GRC blocks
reflect the current USRP2 API in the trunk. Therefore, not all
daughterboard options are available at this time.

The simple and dual usrp source and sink blocks have been polished. The
mux parameter has been removed and the subdevice parameter has been
replaced with antenna selection (RXA, RXB, TX/RX, RX2). Configuration of
the mux and subdevice occurs internally in the simple usrp wrapper
classes. Also, the dual usrp source has complete options for all
possible combinations of RX antennas.

There could be bugs. Enjoy!