Add gRPC streaming middleware. (#95)

* Add gRPC streaming middleware.

Adds a series of gRPC stream middleware for logging, user header propagation and tracing. This is laying groundwork for using gRPC streaming between ingesters and queriers in Cortex.

* Update the vendored versions.
* Pin kuberesolver because the API has changed.

* Always put code under github.com/weaveworks/common;

Dependancies between packages won't work if the code is checked out in github.com/tomwilkie, for instance.  Also, common is a very common name for a repo, so its not uncommon for the repo to be called something like weaveworks-common.

Signed-off-by: Tom Wilkie <tom.wilkie@gmail.com>