Added a NotFound() method so a handler can be specified in the event tha...

Added a new optional method that allows you to handle all the unregistered (404) routes. 

For example:

```
mux := routes.New()
mux.Get("/", handleHome)
mux.Get("/profile/:username([a-zA-Z0-9]+)", handleUserProfile)
mux.NotFound(handle404)
...
```

Also updated the documention (readme).