Also allow a new AcceptContentTypes field to allow the client to ask for
a fallback serialization when getting responses from the server. This
allows a new client to ask for protobuf and JSON, falling back to JSON
when necessary.
The changes to request.go allow error responses from non-JSON servers to
be properly decoded.
The codec factory should support two distinct interfaces - negotiating
for a serializer with a client, vs reading or writing data to a storage
form (etcd, disk, etc). Make the EncodeForVersion and DecodeToVersion
methods only take Encoder and Decoder, and slight refactoring elsewhere.
In the storage factory, use a content type to control what serializer to
pick, and use the universal deserializer. This ensures that storage can
read JSON (which might be from older objects) while only writing
protobuf. Add exceptions for those resources that may not be able to
write to protobuf (specifically third party resources, but potentially
others in the future).
