Added addIdProperty config for ExtJS 4 models.

ExtJS 4 allows model classes to specify an idProperty, which should contain the name of the field that holds the unique id for that model (see the [ExtJs docs](http://docs.sencha.com/extjs/4.2.3/#!/api/Ext.data.Model-cfg-idProperty) for more information. As I found myself readding this property several time to my generated classes I reckoned it would be better to incorporate it into the exporter itself. I disabled idProperty generation by default to preserve backwards compatibility.