Make the Jetpack Account the default if no default WP.com account exists

Also fire off sync of all blogs and notifications.  This should eventually be centralized into a service to handle someone “logging on” to WordPress.com.
Explicit data save after making the Jetpack account the default
Merge pull request #935 from wordpress-mobile/issue/924-jetpack-signin

Make the Jetpack Account the default if no default WP.com account exists