A few changes to PE.h and PE.cpp. Details below:

- Added some macros to make the code independent of the Arduino board used (these macros need to be redefined for other than the standard board, future Richo will have to take care of that).
- Changed the type of "_pinValues" to "unsigned short" in order to handle analog pins (whose range goes from 0 to 1023).
- Now PE::getValue() and PE::setValue() should work independently of pin type and mode.
Removed some comment used for debugging purposes.
Fixed a bug in StackVM::executePrimitive(), the values read from the stack were casted to "unsigned char" instead of "unsigned short" when setting the value.
Also, renamed the variables to something more generic because they might be used for more than just "pin" and "values"