Add support for HS105 protocol

Compared to the HS100 and HS110, where messages sent and received
begin with a four-byte header that can be safely ignored (and set to
zero in the existing code), the HS105 mini-plug requires queries to
have a correct length header matching the message length.

This changeset provides such a header when sending queries. We still
ignore the header when receiving responses from the smart plug.

tplink-smartplug.py

@@ -21,6 +21,7 @@
 #
 import socket
 import argparse
+import struct
 
 version = 0.1
 
@@ -51,7 +52,7 @@ commands = {'info'     : '{"system":{"get_sysinfo":{}}}',
 # XOR Autokey Cipher with starting key = 171
 def encrypt(string):
 	key = 171
-	result = "\0\0\0\0"
+	result = struct.pack(">I", len(string))
 	for i in string:
 		a = key ^ ord(i)
 		key = a