Resolve #24 by adding convert F to C function.

DHT.cpp

@@ -53,6 +53,10 @@ float DHT::convertCtoF(float c) {
 	return c * 9 / 5 + 32;
 }
 
+float DHT::convertFtoC(float f) {
+  return (f - 32) * 5 / 9; 
+}
+
 float DHT::readHumidity(void) {
   float f;
   if (read()) {

DHT.h

@@ -32,6 +32,7 @@ class DHT {
   void begin(void);
   float readTemperature(bool S=false);
   float convertCtoF(float);
+  float convertFtoC(float);
   float computeHeatIndex(float tempFahrenheit, float percentHumidity);
   float readHumidity(void);
   boolean read(void);