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.
I'm not sure we're taking full advantage of this subclass, since `stage(target/name)` could easily live in `Language::Go.stage_deps`, and we're still switching on a `type` parameter in the `resource` DSL method and sniffing the class in `stage_deps`.

If we added a `go_resource` method we could do something like:

```rb
# software_spec.rb
def resource(name, klass=Resource, &block)
  # ...
end

def go_resource(name, &block)
  resource(name, Resource::Go, &block)
end
```
```rb
# resource.rb
class Resource
  class Go
    def download_name
      super.gsub("/", "-")
    end

    def stage(target)
      super(target/name)
    end
  end
end
```