Switch default serializer to JSON.NET

After the blow we got on NDC London, I think it's finally Time To Switch™. We can't really blame SS.Text when we picked it :cry:  
I created a new interface `IObjectSerializer` and pulled the implementation into Hosting. Less dependencies in Core! :wink: There's also some new tests that demonstrated where the SS.Text serializer was failing.

This fixes #288 and #377