Cleanups and tests (#272)

For some reasons there was a dnsserver/middleware.go that defined
the middleware handlers. This code was a repeat from
middleware/middleware.go. Removed dnsserver/middleware.go and replaced
all uses of dnsserver.Middleware with middleware.Middleware.

Added dnsserver/address_test.go to test the zone normalization (and to
improve the test coverage). The deleted file will also improve the test
coverage :)

middleware/kubernetes/setup.go

@@ -42,7 +42,7 @@ func setup(c *caddy.Controller) error {
 		return kubernetes.APIConn.Stop()
 	})
 
-	dnsserver.GetConfig(c).AddMiddleware(func(next dnsserver.Handler) dnsserver.Handler {
+	dnsserver.GetConfig(c).AddMiddleware(func(next middleware.Handler) middleware.Handler {
 		kubernetes.Next = next
 		return kubernetes
 	})