[WIP] Fix bug 858958 - Show dates in user specified timezone

Summary -

1. Set `USE_TZ = True`
2. Replaced `datetime.now()` with `timezone.now()` in wiki.models to make dates timezone-aware
3. Updated `datetimeformat()` template function, to display date in user timezone
4. Update profile template to show localised time

@groovecoder - Is this the right way? Still need to update tests for this.
Also normalize the result to handle cross DST boundaries. See http://stackoverflow.com/a/1422900 or https://docs.djangoproject.com/en/dev/topics/i18n/timezones/#troubleshooting