esp8266: Add dht.py script for high-level control of DHT11/DHT22 sensor.

TODO: should go in a more port-neutral place, like drivers/dht, but at the
moment in relies on specific esp module.

esp8266/scripts/dht.py

+# DHT11/DHT22 driver for MicroPython on ESP8266
+# MIT license; Copyright (c) 2016 Damien P. George
+
+import esp
+
+class DHTBase:
+    def __init__(self, pin):
+        self.pin = pin
+        self.buf = bytearray(5)
+
+    def measure(self):
+        buf = self.buf
+        esp.dht_readinto(self.pin, buf)
+        if (buf[0] + buf[1] + buf[2] + buf[3]) & 0xff != buf[4]:
+            raise Exception("checksum error")
+
+class DHT11(DHTBase):
+    def humidity(self):
+        return self.buf[0]
+
+    def temperature(self):
+        return self.buf[2]
+
+class DHT22(DHTBase):
+    def humidity(self):
+        return (self.buf[0] << 8 | self.buf[1]) * 0.1
+
+    def temperature(self):
+        t = ((self.buf[2] & 0x7f) << 8 | self.buf[3]) * 0.1
+        if self.buf[2] & 0x80:
+            t = -t
+        return t