ading multi-channel tx/rx capabilities
adding first cut at simplified full duplex application

  * not fully capable of simultaneous tx/rx
  * not sure if this is a hardware issue
  * perhaps too much self-interference
  * probably need to dig through some UHD settings
  * need more tx/rx separation?
fullduplex: simplifying interface; still cannot simulataneously tx/rx
ofdmflexframe_[tx|rx]: correcting command-line options
Merge branch 'master' of git://github.com/jgaeddert/liquid-usrp
multichannel[tx|rx]: configuring number of subcarriers etc. in constructor
ofdmflexframetx: validating specified FEC types
multichanneltx: allowing user to specify mod/fec on different channels
ofdmflexframe_rx: adding option to specify rx antenna
asgram: adding fft rate command-line option
narrowband_tx: extending buffer to prevent overflow
ofdmtxrx: adding basic ofdm transceiver class; needs work
ofdmtxrx: simplifying interface
ofdmtxrx: adding more methods (debugging, set antennas)
ofdmtxrx: setting default params, double -> float, adding rx worker
ofdmtxrx: trying to enable transmit mode; needs serious work
ofdmtxrx: fixing transmit bug; forgot to apply software gain
ofdmtxrx: pushing samples through receiver
ofdmtxrx: adding pthread conditions to quickly start/stop receiver
ofdmtxrx: marking start_tx() and stop_tx() as unnecessary