Simplifying microseconds <-> clock cycles conversions. (Rob Tillaart)

This should fix problems with overflows in pulseIn().  It may make millis() slightly less precise for clock speeds that aren't multiple of 1 million, but we don't really support those anyway.

http://code.google.com/p/arduino/issues/detail?id=675

Author: damellis

cores/arduino/Arduino.h

@@ -73,8 +73,8 @@ extern "C"{
 #define noInterrupts() cli()
 
 #define clockCyclesPerMicrosecond() ( F_CPU / 1000000L )
-#define clockCyclesToMicroseconds(a) ( ((a) * 1000L) / (F_CPU / 1000L) )
-#define microsecondsToClockCycles(a) ( ((a) * (F_CPU / 1000L)) / 1000L )
+#define clockCyclesToMicroseconds(a) ( (a) / clockCyclesPerMicrosecond() )
+#define microsecondsToClockCycles(a) ( (a) * clockCyclesPerMicrosecond() )
 
 #define lowByte(w) ((uint8_t) ((w) & 0xff))
 #define highByte(w) ((uint8_t) ((w) >> 8))