Migrate towards LJSONv3. (#785)

* Update the LJSON exporter.

* Update landmark resolver logic to support dicts

Now the resolver/landmark import logic returns dictionaries to
allow for importing LJSON files that map much more closely
to our landmark managers. This means a breaking change but it
is the only way to allow for supporting this. To try and make
the change less painful, the import_landmark_file gains a new
kwarg that allows to choose a single group to return (to more
closely match the old API)

menpo/base.py

@@ -740,6 +740,9 @@ def view_widget(self):
         else:
             return view_widget(self)
 
+    def __str__(self):
+        return 'LazyList containing {} items'.format(len(self))
+
 
 def partial_doc(func, *args, **kwargs):
     r"""