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. 
We actually need to do this at the time of use (or not at all) -- somebody could still set `RefitSettings.JsonSerializerSettings` to null. Json.NET will internally use it's default settings if we give it null, so we can probably do without this.