Add an acceptance test and docs for the down subcommand

Signed-off-by: Daniel Nephin <dnephin@docker.com>

tests/unit/volume_test.py

@@ -0,0 +1,26 @@
+from __future__ import absolute_import
+from __future__ import unicode_literals
+
+import docker
+import pytest
+
+from compose import volume
+from tests import mock
+
+
+@pytest.fixture
+def mock_client():
+    return mock.create_autospec(docker.Client)
+
+
+class TestVolume(object):
+
+    def test_remove_local_volume(self, mock_client):
+        vol = volume.Volume(mock_client, 'foo', 'project')
+        vol.remove()
+        mock_client.remove_volume.assert_called_once_with('foo_project')
+
+    def test_remove_external_volume(self, mock_client):
+        vol = volume.Volume(mock_client, 'foo', 'project', external_name='data')
+        vol.remove()
+        assert not mock_client.remove_volume.called