Added 2FA Support

Added new function _2FA.
Specified that two things that are definitely shown when 2FA is enabled

fbchat/client.py

@@ -231,11 +231,11 @@ class Client(object):
         r = self._cleanPost(LoginURL, data)
         
         # Usually, 'Checkpoint' will refer to 2FA
-        if 'checkpoint' in r.url:
+        if 'checkpoint' in r.url and 'Enter Security Code to Continue' in r.text:
             r = self._2FA(r)
 
         # Sometimes Facebook tries to show the user a "Save Device" dialog
-        if 'save-device' in r.url and 'Enter Security Code to Continue' in r.text:
+        if 'save-device' in r.url:
             r = self._cleanGet(SaveDeviceURL)
 
         if 'home' in r.url: