Added JsonSerializerSettings to refitSettings

Refits will now use a jsonSerializerSettings object to control how to
serialize objects.

This allows the refit settings to be more finely tuned without abusing global state. That way if I want to set that my Refit for this api uses CamelCase, but for another api uses SnakeCase I don't have to worry about them colliding on each other. 
Actually, there is a worse problem. You could in theory do something like...
```csharp
RestService.For<IGitHubApi>("https://api.github.com");
JsonConvert.DefaultSettings = ()=>new JsonSerializerSettings  {
            ContractResolver = new SnakeCasePropertyNamesContractResolver() 
        }
    };
    // I expect to have my api use snake case but it actually won't now.
```
I'll remove the line and change the readme to show you have to new up a settings objects.