Use 'models' tag in response, this allowes us to add new models in future

docs/samples/sync_response.xml

@@ -1,30 +1,30 b''
 <?xml version="1.1" encoding="UTF-8" ?>
 <response>
     <status>success</status>
-    <posts>
+    <models>
         <post>
             <id>id1/1</id>
             <title>13</title>
             <text>Thirteen</text>
             <thread>id1/12</thread>
             <pub-time>12</pub-time>
             <edit-time>13</edit-time>
         </post>
         <post>
             <id>id1/2</id>
             <title>13</title>
             <text>Thirteen</text>
             <pub-time>12</pub-time>
             <edit-time>13</edit-time>
             <tags>
                 <tag>tag1</tag>
             </tags>
         </post>
         <post>
             <id>id2/1</id>
         </post>
         <post>
             <id>id2/5</id>
         </post>
-    </posts>
+    </models>
 </response>