I2C based LCD display and controller

This pull request is the code to implement an I2C port expander based LCD display and interface (called the Panelolu2) in Marlin. It builds on the work done to split the UI and the hardware implementation extending "ultralcd_implementation_hitachi_HD44780.h" to create "ultralcd_implementation_panelolu2.h".

Minor changes have been made to:
Configuration.h - added the option to select the Panelolu2 as a Display
Marlin.pde - ifdefs to include wire.h and LiquidTWI2.h if Panelolu2 is defined, more info on LiquidTWI2 is available here: http://blog.lincomatic.com/?p=965
pins.h - pin allocation within the Sanguinololu section to support the Panelolu2 and fan output on the Adapter board.
ultralcd.cpp - Lines 27-31 ifdefs to include the correct implementation file.

More info available here: http://blog.think3dprint3d.com/2013/02/panelolu2.html

Tony