Added function to get the IMSI

Getting the IMSI is useful for trying to debug issues with peoples subscriptions  in the GSM netwotk and its not printed on the card or anywhere, if the card isn't inserted or hasn't started properly then the command returns ERROR

lib/sim800.py

@@ -213,6 +213,14 @@ def imei():
     else:
         return ""
 
+# Get the IMSI number of the Sim Card
+def imsi():
+    responce = command("AT+CIMI")
+    if (len(responce)>=2):
+        return responce[-2]
+    else:
+        return ""
+
 # Get/Set ringer volume (0-100)
 def ringervolume(level=None):
     # Set the new leve if we have one to set