Refresh WPTableViewController subclasses when app becomes active

viewDidAppear is not fired when bringing the app out of the background.  This observes the notification and fires the refresh accordingly.
Better name for private method to sync automatically
Only refresh table if the view is currently visible
Cleaning up code format
Merge pull request #1181 from wordpress-mobile/issue/1179-refresh-becoming-active

Refresh WPTableViewController coming from background