Abstract calls to digitalWrite and pinMode from the outside "world" by making virtual protected methods

[DrItanium]

After seeing the Adafruit RGBLCDShield I wanted to do the same thing over SPI (using an MCP23S17) but found that the LiquidCrystal library is written to favor code duplication over inheritance. After doing some experiments I found that only digitalWrite and pinMode need to become virtual inside the class. This change does not break the external interface nor internal design in any way.

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