Add ability for models to include themselves

Currently when you add a model type as a field for a model the model you are working with is omitted from the drop down. It would be nice for a model to be able to include itself so that we can build recursive models. e.g.

SomeModel {

field1: string,

field2: object,

field3: SomeModel

}

where field3 would be another instance of SomeModel with fields field1, field2 and field3