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. 
This is actually not a problem either way. I did that so that when you create a new RefitSetting you can do a "simpler" init like I do in the examples. 
```csharp
var gitHubApi = RestService.For<IGitHubApi>("https://api.github.com",
    new RefitSettings {
        JsonSerializerSettings = {
            ContractResolver = new SnakeCasePropertyNamesContractResolver()
        }
    });
```
Works great. But If I remove this line it throws a null ref exception. This requires you to do a `new JsonSerializerSettings`. If Json.net is okay with me passing it null for the settings object, then we are safe.