[model/backend][s]: BackendMemory now works with one and only one dataset at a time (plus document what dataset data looks like).

* It is clear that if Backend is to handle syncing of Documents we need to only be working with one dataset (or: every Document has to reference a dataset -- in which case Backend and Dataset really are just one object ...).

test/model.test.js

@@ -21,8 +21,7 @@ test('new Dataset', function () {
     ]
   };
   // this is all rather artificial here but would make more sense with more complex backend
-  backend = new recline.Model.BackendMemory();
-  backend.addDataset({
+  backend = new recline.Model.BackendMemory({
     metadata: metadata,
     data: indata
     });