Added saving of credentials when self-signed certificates cause challenges to the connection.

Here's a small summary of the changes.
AFAuthenticationAlertview
- If there was an authentication challenge because of a self-signed
certificate, it now checks to see whether that certificate is always
trusted by the user.
- The alert now displays three options when user action is requested on
how to proceed when a self-signed certificate is found.
- If the user selects the 'Always Trust' option, it now saves the
certificate to the KeyChain.

AFXMLRPCClient
- Added a method to delete saved credentials for a self-signed
certificate that was always trusted.

Blog
- When a blog is removed, we now request the API to erase any saved
credentials for this blog, in case a self-signed certificate's
credentials were saved to prevent further authentication challenges.

WPTableViewController
- Fixed a bug where sometimes configureNoResultsView would crash trying
to access self.resultsController.

SidebarViewController
- Fixed a bug where sometimes
processRowSelectionAtIndexPath:closingSidebar: would request a blog
that is out of bounds of the requestController.