Rename RegisterSingle methods to RegisterSingleton.

[dotnetjunkie]

We like to minimize the API and improve confusing parts. The Container.RegisterSingle methods are confusing, because developers sometimes think those methods are the oposite of RegisterMany, while their behavior is completely unrelated.

We would have liked removing those methods altogether in favor of simply using Register overloads that take in a Lifestyle.Singleton, as in Register<HomeController>(Lifestyle.Singleton). We tried this model in the v3 alpha1 release, but this proved to be problematic for users, because it made moving from v2 to v3 extremely hard.

So instead, we rename the RegisterSingle methods to RegisterSingleton. This makes migrating much easier and at the same time makes the API much clearer, because RegisterSingle is still confusing while having RegisterCollection methods at the same time.