vendor dependencies using Godep

We don't have vendoring in place yet. I wish to propose a stopgap solution.

There's a Makefile in project root that exposes two tasks:

`make bundle` copies vendored versions of dependencies to the go workspace.
`make freeze` saves the commit hashes of dependencies (as they exist in the workspace) to the json manifest.

Both tasks depend on `godep`.

Thoughts?

https://github.com/jbenet/go-ipfs/issues/28