Changed feng_auto_acc_len to 2**20 - 1

Which is as large as it can be, given the current firmware
Added print of time.time() as well as timestamp
Removed rogue sleep calls. Added plotting options

Can now use --plot flag to specify a number of
samples to accumulate before plotting (0=don't plot)
Merge branch 'master' of https://github.com/jack-h/ami_correlator_sw