new library for keypads and i2c port expander

Hi, I created a new small library for keypads and i2c port expander PCF8574.

I modified original library of Mark Stanley to do it, I uploaded it to the playground here: http://www.arduino.cc/playground/Main/I2CPortExpanderAndKeypads

Hope it will be useful :)

Works 100% on 4x4 as well, great job Angelito! :slight_smile:

Note to users of PCF8574A, change the address to B00111000 as per page 9 of the datasheet! ;D