Python3 compatibility

[gtnx]

What has bee handled:

• Use of dict.items() instead of dict.iteritems
• Enforce unicode as input of json.loads
• Handling Exception syntax
• Implement __iter__ and __len__ for abstract classes
• Use http.client in python3
• Use urllib.parse.quote and urllib.parse.urlencode in python3
• httplib2 and requests transports have been tested
• twisted transport has not