Vendor missing transitive dependencies

Checking out roshi into a clean `$GOPATH` and running (from the repo root)

    make -C roshi-server

currently fails because the following dependencies of `github.com/prometheus/client_golang` are missing:

    github.com/matttproud/golang_protobuf_extensions
    github.com/prometheus/client_model

This PR fixes that by including both libraries as git subtrees in the `_vendor/` directory.

I don't presume to know what your policy is for vendoring transitive deps, but I had a quick chat with @peterbourgon on Twitter who suggested I submit this PR.

I've vendored both at `master` from their respective repositories. This is presumably safe for  `prometheus/client_model`, which hasn't changed since last November. I considered vendoring `matttproud/golang_protobuf_extensions` at the commit it would have been at when you last updated your copy of `prometheus/client_golang`, but a glance at the commit log suggests that some rather important bugs have been fixed in the intervening period.

After this PR, the following command sequence works on a clean installation with an empty `$GOPATH`, where it didn't before:

    git clone https://github.com/.../roshi
    make -C roshi/roshi-server
    make -C roshi/roshi-walker
    GOPATH="$PWD/roshi/_vendor/:$GOPATH" go test ./roshi/...