Configure the the root handler prefix

I'd like to be able to change /cassette.axd/ to /assets/. I know the `master` branch made `UrlGenerator` public, but for 2.0.0 I've created a hack:

``` csharp
public class CassetteOverridesConfiguration : IConfiguration
{
public void Configure(TinyIoCContainer container)
{
container.Register((c, n) =>
{
var urlGeneratorType = typeof(IUrlGenerator).Assembly.GetTypes().Single(t => t.Name == "UrlGenerator");
return (IUrlGenerator)Activator.CreateInstance(urlGeneratorType,
c.Resolve(), "assets/");
});
}
}
```

And inside my `web.config`:

``` xml

```

Visiting just /assets/ displays the diagnostic page. However, when attempting to load the assets, it 404s.

And this makes sense since `RawFileRequestRewriter`, `CassetteHttpModule` are hard coded to use `cassette.axd`.