Notifications refreshing automatically too often

Fixes #1206 & #955 

NotificationsViewController was firing off a call to refresh too often.  WPTableViewController was updated to be a bit more aware of the right situation, but the call to refresh in the subclass NotificationsViewController viewDidAppear wasn't altered/removed.  

NotificationsViewController will now only automatically refresh on viewDidAppear if the right amount of time has passed and if the user is connected to the Internet.  The user can still force a refresh and they'll be presented with the error message about being offline.

In addition, we're not going to clear the badge count on the app unless Note fetchNotificationsSince:success:failure succeeds.