Somewhat torn on this one. On the one hand it better reflects the utility when you use the initializer but once you do that it makes the static accessor methods for the default transformers feel out of place.

If there was a reason to have a `RKValueTransformer` parent class I'd factor it into a class hierarchy along those lines… but I don't see a great reason to provide an empty subclassable implementation that basically does the same thing, but uses methods for implementation.