A way to simplify customized serialization usage

A **static Dictionary\<Type, JsonConverter\>** was added to JsonConvert class, which the "final" **SerializeObjectInternal** and **DeserializeObject** method will automaticlly get the right **JsonConverter** from, if any.

So, by registering a custom JsonConverter class in advance, the serialization/deserialization code need **not** to pass a JsonConverter everytime any more.

This idea comes from MongoDB serialization mechanism, which is very convenient.