Runtime command pinned out! Re-organized coolant pins.

- Pinned out cycle start(A2), feed hold(A1), and reset(A0) runtime
commands. These pins are held high with the internal pull-up resistor
enabled. All you have to do is connect a normally-open switch to the
pin and ground. That's it.

- Moved the coolant control pins to A3 (and the optional mist control
to A4).

- Moved all of the MASK defines into the config.h file to centralize
them.

limits.h

@@ -21,7 +21,6 @@
 #ifndef limits_h
 #define limits_h 
 
-#define LIMIT_MASK ((1<<X_LIMIT_BIT)|(1<<Y_LIMIT_BIT)|(1<<Z_LIMIT_BIT)) // All limit bits
 #define MICROSECONDS_PER_ACCELERATION_TICK  (1000000/ACCELERATION_TICKS_PER_SECOND)
 
 // initialize the limits module