Facebook sso pending share fix

1. Fixes bug where pending action couldn't be restored after SSO login. This happens because pendingAction is set to SHKPendingShare in SHKSharer after [self saveItemForLater:self.pendingAction]; is called in SHKFacebook.

2. If developer uses FacebookSDK for other purposes in other parts of the app, calling openActiveSession from two places can cause crash or other unexpected behaviour. This fix opens session only if it is needed. It's better to add the separate method that checks if there is a stored item in the user defaults. Currently the restore method also removes the stored data. So the code looks a little bit dirty