Add basic Galileo support

Add digitalWrite support for Galileo Gen1 board.
Refactor common code into higher hierarchy class and make GPIOGalileo and
GPIOGalileoGen2 classes inherit from it.
Add digitalRead support for Galileo

Add muxing for input mode.
Move digitalRead function to base class.
Add analogRead support for Galileo

Add muxing for analog input mode.
Move analogRead function to base class.
Add analogWrite support for Galileo

Add muxing for pwm mode.
Move analogWrite, setPWMPeirod and cleanup function to base class.
Add more specific constructors to match needs of each subclass, specially with
pwm management.
Add comments to examples

Add comments to example scripts stating these will work on GalileoGen2 out of
the box but running them on Galileo is just a matter of changing the import
from wiringx86.
Update documentation with Galileo support