Added isPartLoaded and getRegisteredParts to $translatePartialLoader

I needed these changes to write a more robust translate-cloak. The problem is $translateLoadingSuccess is fired on every part load. Therefore current implementation of translate-cloak doesn't work correctly.

So I added a couple of methods and wrote my own cloaker.
@mruoss I think what he tries to say is that at the time when this method is called, you actually don't put your app in an invalid state and therefore a check like this isn't necessary. If you have a method that **sets** something instead of getting something, it makes sense to check the value and throw and error when the given value puts the app in an invalid state.