Move logger to dash namespace, add stream handler once.

[T4rk1n]

• Moved the logger namespace to dash.dash, as library logger it should be on that namespace instead of the user app.
• Fix multiple log handler added unconditionally to the logger, resulting in duplicate log message.
• Add add_log_handler=True to Dash init, if you don't want a stream handler at all.

Fix #1099, Fix #756

[alexcjohnson]

Thanks, nicely done.
💃 with a changelog entry