UHD support for tuning on UmTRX

Hi all,

I've just pushed changes to the 'fairwaves/umtrx' branch which allow UHD to actually control UmTRX tuning. This means that now we don't need to calculate Rx/Tx frequencies by hands to run OpenBTS - it will tune automatically. I've updated the wiki to reflect those changes as well.

Changes are based on the Sergey Kostanbaev's work to move all our LMS6002 control from umtrx_lms.py script to the UHD. The next most important things to get working there are: * gains control aka VGA1/VGA2 in LMS * antenna control aka PA/LNA selection in LMS * bandwidth filtering control aka LPF filters in LMS * automatic calibration utilities