Merge pull request arduino#31 from jamadagni/patch-1

Add `bitToggle` macro to complement `bitSet` etc

Author: aentinger

cores/arduino/Arduino.h

@@ -111,6 +111,7 @@ void yield(void);
 #define bitRead(value, bit) (((value) >> (bit)) & 0x01)
 #define bitSet(value, bit) ((value) |= (1UL << (bit)))
 #define bitClear(value, bit) ((value) &= ~(1UL << (bit)))
+#define bitToggle(value, bit) ((value) ^= (1UL << (bit)))
 #define bitWrite(value, bit, bitvalue) ((bitvalue) ? bitSet(value, bit) : bitClear(value, bit))
 
 // avr-libc defines _NOP() since 1.6.2