Add go dependency helper

This makes a couple minor improvements to make tracking go deps as resources easier:

* Resource names now translate `/` into `-` in the donwnload path, so resource names can contain full go package names, e.g. `github.com/neeee/go-fftw`
* Added a small godep staging helper, so godeps can be staged in a one-liner to replace `go get`

mpdviz was updated as an example.