Device support of the Xiaomi PM2.5 Air Quality Monitor introduced. Fixes

 #70.

miio/__init__.py

@@ -14,6 +14,7 @@
 from miio.chuangmi_ir import ChuangmiIr
 from miio.fan import Fan
 from miio.wifispeaker import WifiSpeaker
+from miio.airqualitymonitor import AirQualityMonitor
 from miio.yeelight import Yeelight
 from miio.device import Device, DeviceException
 from miio.discovery import Discovery

miio/airqualitymonitor.py

@@ -0,0 +1,65 @@
+import logging
+from collections import defaultdict
+from .device import Device
+
+_LOGGER = logging.getLogger(__name__)
+
+
+class AirQualityMonitorStatus:
+    def __init__(self, data):
+        # ['power': 'on', 'aqi': 34, 'battery': 0, 'usb_state': 'on']
+        self.data = data
+
+    @property
+    def power(self) -> str:
+        return self.data["power"]
+
+    @property
+    def is_on(self) -> bool:
+        return self.power == "on"
+
+    @property
+    def usb_power(self) -> bool:
+        return self.data["usb_state"] == "on"
+
+    @property
+    def aqi(self) -> int:
+        return self.data["aqi"]
+
+    @property
+    def battery(self) -> int:
+        return self.data["battery"]
+
+
+class AirQualityMonitor(Device):
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
+    def status(self):
+        """Return device status."""
+
+        properties = ['power', 'aqi', 'battery', 'usb_state']
+
+        values = self.send(
+            "get_prop",
+            properties
+        )
+
+        properties_count = len(properties)
+        values_count = len(values)
+        if properties_count != values_count:
+            _LOGGER.debug(
+                "Count (%s) of requested properties does not match the "
+                "count (%s) of received values.",
+                properties_count, values_count)
+
+        return AirQualityMonitorStatus(
+            defaultdict(lambda: None, zip(properties, values)))
+
+    def on(self):
+        """Power on."""
+        return self.send("set_power", ["on"])
+
+    def off(self):
+        """Power off."""
+        return self.send("set_power", ["off"])