Generic interface support

When using something like Web API, it's a fairly common pattern to have a whole stack of CRUD rest services. Refit now supports these, allowing you to define a single API interface with a generic type:

```csharp
public interface IReallyExcitingCrudApi<T, in TKey> where T : class
{
    [Post("")]
    Task<T> Create([Body] T paylod);

    [Get("")]
    Task<List<T>> ReadAll();

    [Get("/{key}")]
    Task<T> ReadOne(TKey key);

    [Put("/{key}")]
    Task Update(TKey key, [Body]T payload);

    [Delete("/{key}")]
    Task Delete(TKey key);
}
```

Which can then be used like this:

```csharp
// The "/users" part here is kind of important if you want it to work for more 
// than one type (unless you have a different domain for each type)
var api = RestService.For<IReallyExcitingCrudApi<User, string>>("http://api.example.com/users"); 
```


---
This fixes #63.

I will update the docs to something like the above if you're happy with the changes.
:arrow_down: