Use `godep` as the dependency management tool

Both from a discussion at GDS and from recent community posts there
finally seems to be some consistency on how to do dependency
management in Go.

The other viable option was `third_party.go` but the CoreOS folks have
since publicly stated[1] they're going to prefer using Godep for end
user Go projects.

[1] https://coreos.com/blog/godep-for-end-user-go-projects/