Add a JSONPojoConvertor and enhance JSONObjectConvertor to do caching

Add a JSONPojoConvertor and enhance JSONObjectConvertor to do caching.

• Converts POJOs to JSON and vice versa.
• The key difference:
• - returns the actual object from Convertor.fromJSON (JSONObjectConverter returns a Map)
• - the getters/setters are resolved at initialization (JSONObjectConverter resolves it at runtime)
• - correctly sets the number fields

The JSONObjectConvertor could act as the default convertor and act as a cache of pojo convertors.