etcdserver: introduce Server interface

This changes etcdserver.Server to an interface, with the former Server
(now "server") becoming the canonical/production implementation. This will
facilitate better testing of the http server et al with mock implementations
of the interface. It also more clearly defines the boundary for users of the
Server. A NewServer convenience function is added to create the Server with
some default/production values.
I think all errors should return in json format based on issue #726 here.
just a reminder, and i am fine to keep it as this for now.