Changed enum implementation to enable short syntax in Swift

Before I always needed to put the full name of a `CSNotificationViewStyle` in Swift which lead to very long names. Since Swift there's a shorter syntax which was introduced exactly for cases like this. In order to enable the shorter syntax it is needed to use the `NS_ENUM` method, which I have done with this pull request.

**Important:**
All clients using `CSNotificationViewStyle` in Swift must replace `CSNotificationViewStyleError` with `.Error` and `CSNotificationViewStyleSuccess` with `.Success`.