sample_grabber now saves bytes it receives as is rather than doing
mapping itself
Changed saving of samples - now packed into bytes in Piksi format, ie
  7:5 = sample 0 sign, sample 0 mag high, sample 0 mag low
  4:2 = sample 1 sign, sample 1 mag high, sample 1 mag low
  1 = unused
  0 = sample 0 FPGA FIFO overflow flag
Assumes data received from HDL is in the format
  7:5 = sample sign, sample mag high, sample mag low
  4:1 = unused
  0 = sample FPGA FIFO overflow flag
Merge branch 'master' of https://github.com/cbeighley/piksi_sample_grabber

Conflicts:
	sample_grabber.c