Add support for collections.OrderedDict

[gonvaled]

In my json decoder, I usually use json.loads(json_str, object_pairs_hook=OrderedDict), so that key-ordering is preserved. But the json_schema_generator does not support this:

json_schema_generator.schema_types.JsonSchemaTypeNotFound: There is no schema type for <class 'collections.OrderedDict'>.

[wizzat]

Do you still need this?