Added stub for "get_all_users" user service.

This is for SGA issue 115:

[issue 115](mitodl/edx-sga#115)

xblock/reference/user_service.py

@@ -20,6 +20,16 @@ def get_current_user(self):
         """
         raise NotImplementedError()
 
+    def get_all_users(self, **kwargs):
+        """
+        This is default, example implementation.  Anything real needs to override
+
+        This is expected to return a list instance of XBlockUser instances.
+
+        **kwargs is intended to be used for filtering options, such as by course.
+        """
+        raise NotImplementedError()
+
 
 class XBlockUser(object):
     """

xblock/test/test_user_service.py

@@ -17,6 +17,9 @@ def __init__(self, user):
     def get_current_user(self):
         return self.user
 
+    def get_all_users(self):
+        return [self.user]
+
 
 def test_dummy_user_service_current_user():
     """
@@ -34,6 +37,17 @@ def test_dummy_user_service_current_user():
     assert_is_instance(current_user.opt_attrs, collections.Mapping)
 
 
+def test_dummy_user_service_all_users():
+    """
+    Tests that get_all_users() works on a dummy user service.
+    """
+    user = XBlockUser(full_name="tester")
+    user_service = SingleUserService(user)
+    all_users = user_service.get_all_users()
+    assert_is_instance(all_users, list)
+    assert_equals(user, all_users[0])
+
+
 def test_dummy_user_service_exception():
     """
     Tests NotImplemented error raised by UserService when not instantiated with kwarg get_current_user