Implement 'labels' option

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

compose/container.py

@@ -79,6 +79,10 @@ class Container(object):
         return ', '.join(format_port(*item)
                          for item in sorted(six.iteritems(self.ports)))
 
+    @property
+    def labels(self):
+        return self.get('Config.Labels') or {}
+
     @property
     def human_readable_state(self):
         if self.is_running: