Use a more standardized project layout

In #2, mrngm made the point that we should move to a more
standardized project structure. This commit does exactly that.
The new project structure is based on the repository listed at
<https://github.com/golang-standards/project-layout>.

Fixes #2.

go.mod

@@ -3,6 +3,7 @@ module gitea.hashru.nl/dsprenkels/rushlink
 go 1.12
 
 require (
+	github.com/go-bindata/go-bindata v3.1.2+incompatible // indirect
 	github.com/gorilla/mux v1.7.3
 	github.com/pkg/errors v0.8.1
 	github.com/prometheus/client_golang v1.1.0