Consolidate go dependencies into a root Godeps/ dir

There are several separate `Godeps/` directories in the Deis project for mostly historical reasons. This unifies them into a single root `Godeps/` dir. The `godep` tool searches upward through parent directories until it finds a `Godeps/` dir, so few changes were necessary.

Docker builds cannot reference files outside the build context, so this change copies the `Godeps/` directory into the required locations when necessary for a `make build` target. This is fast, and the copied files are masked from version control, but I'm open to a more elegant solution.

This *should* be one commit, but it's difficult to review such a large diff, so all the moving of files and updating of Godeps is in the first commit, and the salient changes to Makefiles for review are in the second.

Closes #2699.