Dictionary serialization fix

Fixes #11, although I'm not entirely happy with the approach.

SimpleJson handles IDictionary<string, object> specifically. If you try to serialize a property of type IDIctionary<string, Blah> you opt out of that. I could not see any obvious way to account for that besides changing the model properties to IDictionary<string, object> - which is a bit shit.

Unless anyone knows a technique for handling this within the SerializerStrategy, then the only other option is to use an alternative serializer.