Update to add timezone support.

[benmurden]

In order to avoid timezone support warnings, change default values for metric items and gauges to timezone aware equivalents from django.utils.timezone.

This requires some changes to move_to_mixpanel command so timestamps can be calculated correctly. To support more platforms, changed the use of strftime('%s') to a more compatible implementation that closely follows the Python 3 implementation of timestamp().

Added timedelta_total_seconds() to allow continued support for Python 2.6.

Added tests for timestamp utility functions.

[benmurden]

Seems to be breaking support for Django 1.3, where timezone doesn't exist.