Add a raw path handler to util

Supports handling paths like "/foo/bar%2Fbaz", which Go 1.X does not
correctly handle according to RFC (https://code.google.com/p/go/issues/detail?id=3659).
A concrete case for this is REST resources whose ID contains slashes which
could include Docker repository/registry names, URLs, and path segments.
Centralize path magic in apiserver into the New method

Make OperationHandler and WatchHandler properly encapsulate their
concerns and make them not depend on the global path
Support raw URL paths in the apiserver