Wunderground sensor with alerts exceeds API limits (#4070)

* Fixes issue #4067 - Wunderground sensor with alerts exceeds API limits

 To avoid hitting the max limit of 500 calls per day, this patch keeps weather conditions being updated each 5 minutes
  and weather advisories each 15 minutes.

 This formula will result the following:

   conditions -> 300 seconds -> 5 minutes -> 12 req/h -> 288 req/day
   alerts -> 900 seconds -> 15 minutes -> 4 req/h -> 96 req/day

* Using timedelta in minutes instead seconds

homeassistant/components/sensor/wunderground.py

@@ -25,7 +25,8 @@ _LOGGER = logging.getLogger(__name__)
 CONF_ATTRIBUTION = "Data provided by the WUnderground weather service"
 CONF_PWS_ID = 'pws_id'
 
-MIN_TIME_BETWEEN_UPDATES = timedelta(seconds=300)
+MIN_TIME_BETWEEN_UPDATES_ALERTS = timedelta(minutes=15)
+MIN_TIME_BETWEEN_UPDATES_OBSERVATION = timedelta(minutes=5)
 
 # Sensor types are defined like: Name, units
 SENSOR_TYPES = {
@@ -187,7 +188,7 @@ class WUndergroundData(object):
 
         return url + '.json'
 
-    @Throttle(MIN_TIME_BETWEEN_UPDATES)
+    @Throttle(MIN_TIME_BETWEEN_UPDATES_OBSERVATION)
     def update(self):
         """Get the latest data from WUnderground."""
         try:
@@ -202,7 +203,7 @@ class WUndergroundData(object):
             self.data = None
             raise
 
-    @Throttle(MIN_TIME_BETWEEN_UPDATES)
+    @Throttle(MIN_TIME_BETWEEN_UPDATES_ALERTS)
     def update_alerts(self):
         """Get the latest alerts data from WUnderground."""
         try: