Add factory methods to return UIAlertView without showing it

The factory methods to build and show the configured `UIAlertView` in one shot are pretty handy. In some cases though I'd like to build the alert view but don't show it straightaway.

I know that this is possible using the `initWithTitle:message:delegate:cancelButtonTitle:otherButtonTitles:` method and setting the `tapBlock` afterwards, but I like the simplicity of the factory method.