Adding pin and sample code for LCDs with backlights using I2C/SPI LCD Backpack 

The Adafruit_CharLCD library did not have default code for handling LCDs with backlights. I have set a new default (pin_b) in the initializer to pin 19 for RPi.GPIO and pin 7 in the sample code using the I2C/SPI Backpack with Adafruit_MCP230xx chip.