added type property for catalogs and collections

Addresses the changes in radiantearth/stac-spec#971

Author: volaya

pystac/catalog.py

@@ -366,6 +366,7 @@ def to_dict(self, include_self_link=True):
             links = filter(lambda l: l.rel != 'self', links)
 
         d = {
+            'type': self.STAC_OBJECT_TYPE.value.title(),
             'id': self.id,
             'stac_version': pystac.get_stac_version(),
             'description': self.description,