Exposed DataSnapshot so it can be used

Author: rhodgkins

src/index.js

@@ -9,3 +9,4 @@ exports.MockFirestore = require('./firestore');
 exports.MockStorage = require('./storage');
 exports.MockMessaging = require('./messaging');
 exports.DeltaDocumentSnapshot = MockFirestoreDeltaDocumentSnapshot.create;
+exports.DataSnapshot = require('./snapshot');