Provide timestamps in UTC

Instead of using local time, return the image timestamp in UTC.  This is
better for interoperating with other systems that need the time, and
probably already use UTC.

This requires converting the timestamp provided by the SDK to UTC before
building the DateTime object.
DECREF info values after adding to dictionary

As in d272a7f, DECREF values added to the info dictionary.  This was
missed in that commit.

Adding to the dictionary will INCREF the values, and we do not need them
anymore, so DECREF them.