Add a generic proxier

To proxy traffic to anything that implements ResourceLocation.
Currently, this is only services. This is easily extensible to minions
(would supercede existing mechanism) and pods.
We do 404 in the other handlers to indicate that you tried to access something that doesn't exist (the handler is defined for all rest storage objects today, rather than just those that implement redirector).  Not every RESTStorage has to implement Redirector.

However, just in general, doing these interface tests here is probably the wrong pattern - we may want to eventually change the map[string]RESTStorage to be a map[string]StructForMultipleInterfaces