Added R/W byte for chips w/o registers (eg PCF8574)

Hi,

added possibility to communicate with dumb chips (like the old PCF8574) who do not have registers.
Should also give the possibility to read again and again the same register for other chips. 

names come from recent commit of Adafruit-Raspberry-Pi-Python-Code/Adafruit_I2C/Adafruit_I2C.py