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.
Leaving as 404. It's expected that not all objects implement the Redirector interface, and that means 404, it's not a server error.

In the future we may wish to have ProxyHandler take a map of Redirectors instead of a map of RESTStorage objects, and do this assertion elsewhere, which would have the same net effect (result in 404).