Added support for ExtendedData elements

Some changes that will allow `ExtendedData` elements to be parsed. This element can contain `Data` elements as well as `SchemaData` elements. 

*Classes that were added:*
* `SimpleKMLExtendedData` class that models the `ExtendedData` element in KML, extends `SimpleKMLObject`
* `SimpleKMLSubData` abstract class that extends `SimpleKMLObject`
* `SimpleKMLData` class that models the `Data` element in KML, extends `SimpleKMLSubData`
* `SimpleKMLSchemaData` class that models the `SchemaData` element in KML, extends `SimpleKMLSubData`

https://developers.google.com/kml/documentation/kmlreference?hl=nl#extendeddata