Add WrapFunc for convenient routing

```
n := negroni.Classic()

r := mux.NewRouter()
r.Handle("/admin", negroni.New(Middleware1, Middleware2, AdminHandler))
n.UseHandler(r)

n.Run(":3000")
```

You gave this example in this issue: https://github.com/codegangsta/negroni/issues/25

I was wondering if we could add a <code>WrapFunc()</code> to shorten the <code>AdminHandler</code> a bit.

**Result:**

```
r.Handle("/admin", negroni.New(Middleware1, Middleware2, 
                               negroni.WrapFunc(func(w http.ResponseWriter, r *http.Request) {

    w.Write([]byte("admin only"))
})))
```

**As opposed to..**
```
r.Handle("/admin", negroni.New(Middleware1, Middleware2, 
                               negroni.Wrap(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {

    w.Write([]byte("admin only"))
}))))
```

Just a thought.