Add experimental support for smartplugs

This adds a set_power() call that should work for the SP2 - unsure whether
it will work for the SP3.

README.md

@@ -43,3 +43,8 @@ Obtain sensor data from an A1:
 ```
 data = devices[0].check_sensors()
 ```
+
+Set power state on an SP2/SP3 (0 for off, 1 for on):
+```
+devices[0].set_power(1)
+```

broadlink/__init__.py

@@ -180,6 +180,12 @@ class device:
     response = self.cs.recvfrom(1024)
     return response[0]
 
+  def set_power(self, state):
+    packet = bytearray(8)
+    packet[0] = 2
+    packet[4] = state
+    self.send_packet(0x6a, packet)
+
   def send_data(self, data):
     packet = bytearray([0x02, 0x00, 0x00, 0x00])
     packet += data