Storing utc datetime values for auto_now_add and auto_now

[kishorpawar]

I went through your code, and found that when auto_now_add is set to True you store datetime.now() .
This shall fail in timezone oriented app. Though mostly applications are hosted on cloud servers and they are configured for utc timezone. This library won't fail when server is in utc timezone. This library will definitely fail in development environment if development machine is not set to use utc timezone.

Though your code can store timezone aware datetimes, but auto_now_add will be useless in timezone oriented app.

Hope you would fix this.

Thanks for brilliant hard work.