Add a DSL Redirect

[tchssk]

This pull request adds a new DSL to define a HTTP redirect.

The DSL can be used like below:

var _ = Service("service", func() {
	Method("method", func() {
		HTTP(func() {
			GET("/resources")
			Redirect("/redirect/dest", StatusMovedPermanently)
		})
	})
})

var _ = Service("service", func() {
	Files("/file.json", "/path/to/file.json", func() {
		Redirect("/redirect/dest", StatusMovedPermanently)
	})
})

The generated server handler initalizer for the HTTP endpoint redirect is:

func NewMethodHandler(
	endpoint goa.Endpoint,
	mux goahttp.Muxer,
	decoder func(*http.Request) goahttp.Decoder,
	encoder func(context.Context, http.ResponseWriter) goahttp.Encoder,
	errhandler func(context.Context, http.ResponseWriter, error),
	formatter func(err error) goahttp.Statuser,
) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		ctx := context.WithValue(r.Context(), goahttp.AcceptTypeKey, r.Header.Get("Accept"))
		ctx = context.WithValue(ctx, goa.MethodKey, "Method")
		ctx = context.WithValue(ctx, goa.ServiceKey, "Service")
		http.Redirect(w, r, "/redirect/dest", http.StatusMovedPermanently)
	})
}

and the generated server mounter for the file server redirect is:

func Mount(mux goahttp.Muxer) {
	MountPathToFileJSON(mux, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		http.Redirect(w, r, "/redirect/dest", http.StatusMovedPermanently)
	}))
}

[raphael]

This looks great, thank you for the PR! Just one question: it seems the DSL func argument to Redirect is only there to allow for using Meta. And it doesn't like any metadata attached to redirects would be used anywhere at this point. Would it make sense to simplify and remove the DSL argument and the Meta field or do you have a use for it?