Implemented Wakeup from sleep with interrupt #95

Can use the following method for wakeup from sleep:
Spark.sleep(uint16_t wakeUpPin, uint16_t edgeTriggerMode);

Example:
Here calling this function will immediately stop the Core and then if a High voltage/Logic 1 is applied on D2 pin, Core will wakeup from sleep. 
Spark.sleep(D2, RISING);
Spark.sleep(D2, RISING, 3600);//Here the Core will wakeup after 1  hour if no interrupt arrive

SImilarly if the edge triggered mode is set to "FALLING" applying Low voltage/Logic 0 will wakeup from sleep
Spark.sleep(D2, FALLING);
Spark.sleep(D2, FALLING, 3600);


Might be a good idea not to reset the register until the parameters have been validated a few lines down.