Add support for declaring named volumes in compose files

* Bump default API version to 1.21 (required for named volume management)
* Introduce new, versioned compose file format while maintaining support
  for current (legacy) format
* Test updates to reflect changes made to the internal API

Signed-off-by: Joffrey F <joffrey@docker.com>

compose/volume.py

@@ -0,0 +1,19 @@
+from __future__ import unicode_literals
+
+
+class Volume(object):
+    def __init__(self, client, project, name, driver=None, driver_opts=None):
+        self.client = client
+        self.project = project
+        self.name = name
+        self.driver = driver
+        self.driver_opts = driver_opts
+
+    def create(self):
+        return self.client.create_volume(self.name, self.driver, self.driver_opts)
+
+    def remove(self):
+        return self.client.remove_volume(self.name)
+
+    def inspect(self):
+        return self.client.inspect_volume(self.name)