This allows forcing a read, even if the previous read was less than 2
seconds ago. This is useful in cases where the millis() timer is not
reliable, such as when sleeping. In this case, it is up to the caller to
ensure that at least 2 seconds elapse between calls with force set to
true.
Added constructor parameter to allow for different timing needs. It has
the a default value of 6 which is the original value, so it should be
backwards compatible
Added a basic #ifndef DHT_H to the dht.h file so that if you have a
larger program and want to access the sensor from a different file, you
don't import it twice.
