adding the ability to re-route the standard resource member actions

I love how this library guides me naturally towards a restful way of thinking.

That said, there are some endpoints that I constantly want to re-route in favor of more human-readable and commonly discoverable urls:

<table>
<tr><th>method</th><th>route</th><th>controller#action</th></tr>
<tr><td>GET</td><td>~/signup</td><td>account#new</td></tr>
<tr><td>GET</td><td>~/login</td><td>session#new</td></tr>
<tr><td>POST</td><td>~/login</td><td>session#create</td></tr>
</table>

This branch includes my attempt to refactor the path part of the ResourceMapper<T> and expose overrides that support this use case:

```csharp
public class mapping_resource_with_reroute_overrides : base_context
{
    static ResourceMapper<SessionsController> _mapper;
    
    Establish context = () =>
    {
        _mapper = new ResourceMapper<SessionsController>();
        _mapper.ReRoute(c => c.New = "signup");
    };

    Because of = () => _mapper.RegisterRoutes(routes);

    It should_map_get_new = () => "~/signup".WithMethod(HttpVerbs.Get).ShouldMapTo<SessionsController>(x => x.New()).WithName("new_session");
}
```

Feedback is welcome, as is the decision whether or not this change is in keeping with the philosophy of the project.