Fix file serving

Since we no longer use the global `http.DefaultServerMux` but our own, the `ServeStatic()` method was broken.

This PR fixes this (and a few other minor things):
* Added new example for file serving.
* Fixed logaccess reporting to cache initially requested method+url (`http.StripPrefix` changes the URL)
* AccessLogger is now only added to each router once, instead of each handler
* ServeStatic() uses the internal routers instead of the net/http package.
That is kind of `fileserver-example`.