Improvements to dependency management

[tgross]

This PR tries to make the dependency management story easier to understand while still allowing our upcoming subpackages to be "go gettable" for #83. This also assists me in prepping for #27.

The specific changes are:

• Eliminated the second makefile for inside the container, which I found hard to debug because of redundant env vars.
• Add add-dep makefile target that will use go get to fetch the dependency and record it via Godeps save function; this makes it easier to add new dependencies correctly.

The new add-dep target works like this in practice:

DEP=github.com/prometheus/client_golang/prometheus PKG=metrics make add-dep
docker run --rm \
            -v /Users/tim.gross/src/tgross/containerbuddy:/go/cb \
                -v /Users/tim.gross/src/tgross/containerbuddy:/go/cb/src \
                -e GOPATH=/go/cb/src/metrics \
                -w /go/cb/src/metrics \
                containerbuddy_build go get github.com/prometheus/client_golang/prometheus
docker run --rm \
            -v /Users/tim.gross/src/tgross/containerbuddy:/go/cb \
                -v /Users/tim.gross/src/tgross/containerbuddy:/go/cb/src \
                -e GOPATH=/go/cb/src/metrics \
                -w /go/cb/src/metrics \
                containerbuddy_build godep save
mv metrics/src metrics/vendor

cc @justenwalker @misterbisson

[tgross]

Oops... I appear to have broken it during cleanup. Lemme fix. I dropped a makefile target. Next run should fix it.

[tgross]

Ok, after a couple of dumb mistakes and a rebase to hide the bodies, this should be good-to-go. 😀

[misterbisson]

LGTM.

@justenwalker are you following along with these changes?

🏡 🚶

[justenwalker]

@misterbisson Yeah, I've been looking a them - haven't had much time to verify them locally but they LGTM

Very nice cleanup efforts @tgross !