Add top-level API

Add the top-level class that 99% of people will use to actually do stuff. Here's how it works:

```cs
public interface IGitHubApi
{
    [Get("/users/{username}")]
    Task<User> GetUser(string userName);
}

var service = RestService.For<IGitHubApi>("https://api.github.com");
var result = await service.GetUser("octocat");
```