Allow gain to auto-scale if a current overflow occurs

[chrisb2]

The idea is to make it even easier to use this library by automatically increasing the gain if a current overflow is detected at the current gain. Current overflow would only occur when the gain reaches the maximum value support by the INA219.

Auto-scaling of gain would be the default configuration, so to measure voltage upto 26V and current upto 3.2A (with std 0.1 Ohm shunt) only code similar to the following would required:

SHUNT_OHMS = 0.1
ina = INA219(SHUNT_OHMS)
ina.configure()

try:
    print "Bus Voltage: %.3f V" % ina.voltage()
    print "Bus Current: %.3f mA" % ina.current()
except CurrentOverflowException"
    print "Current greater than device capability"