move has_emeter implementation from SmartDevice to SmartPlug, avoid using features() internally (#93)

* move has_emeter implementation from SmartDevice to SmartPlug, avoid using features() internally

* add stacklevel to deprecation warnings to see where they are really called

* make tests pass on a real device. if PLUG_IP is not None, the tests will be run on a device at the defined IP address

pyHS100/smartplug.py

@@ -80,6 +80,16 @@ class SmartPlug(SmartDevice):
         else:
             raise ValueError("State %s is not valid.", value)
 
+    @property
+    def has_emeter(self):
+        """
+        Returns whether device has an energy meter.
+        :return: True if energy meter is available
+                 False otherwise
+        """
+        features = self.sys_info['feature'].split(':')
+        return SmartDevice.FEATURE_ENERGY_METER in features
+
     @property
     def is_on(self) -> bool:
         """