Use compact JSON encoding for Python objects

When encoding Python objects to JSON, we currently use json.dumps() with default arguments. This produces JSON encoded with spaces separating fields. This is somewhat wasteful from a memory / disk / network utilisation point of view.

More preferable would be to use a "compact" encoding which removes all spaces - this can be achieved using separators=(',', ':') or similar (see http://docs.python.org/2/library/json.html).