Save reading, load reading, small bitcount calculator

	#./rfidler.py /dev/RFIDler 'set tag askraw' 'potset h 150' 'set fc 745' 'set rate 33' dump 6000
	sending 'SET TAG ASKRAW'
	sending 'POTSET H 150'
	sending 'SET FC 745'
	sending 'SET RATE 33'
	Wrote dump to dump_20141121-075357.xml

- Added functionality to plot from a previously saved reading:

	#./rfidler.py /dev/RFIDler load dump_20141121-075357.xml

- Added some logic to print out bit periods, i.e. the number of samples before the next toggling (not the true bit period, just to help determine the true bit period)