ref(*): rewrite the slugbuilder in Go

[arschles]

This PR is experimental, mostly to determine the scope of the rewrite. For now, the rewrite may only be for parts of slugbuilder to keep the scope of this PR down.

[arschles]

@smothiki @kmala @gabrtv @mboersma thoughts on closing this? The only meaningful semantic change that we'd see with a Go rewrite is that we'll be accessing object storage with the github.com/deis/distribution package, instead of the object-storage-cli binary.

[smothiki]

I'm Ok closing this as we are already using storage cli where most of the code falls