Add config option for where to redirect mobile/phone browsers

[hammerlabs]

New code in my_base.php would look like:

$config["mobile_redirect_url"] = "/mobile";

and then in device_redirect.php:

redirect( $this->config->item( 'mobile_redirect_url' ) );

I'd also like to optionally set all controllers on the site to force mobile detection and redirection. Perhaps we leave that turned off for development, but enabled for production and testing environments only.