New model id format

docs/samples/request_get.xml

@@ -1,7 +1,7 b''
 <?xml version="1.1" encoding="UTF-8" ?>
 <request version="1.0" type="get">
     <model version="1.0" name="post">
-        <id>id1/1</id>
-        <id>id2/56</id>
+        <id key="id1" local-id="1" />
+        <id key="id1" local-id="2" />
     </model>
 </request>

docs/samples/response_get.xml

@@ -3,7 +3,7 b''
     <status>success</status>
     <models>
         <model name="post">
-            <id>id1/1</id>
+            <id key="id1" local-id="1" />
             <title>13</title>
             <text>Thirteen</text>
             <thread>id1/12</thread>
@@ -15,16 +15,16 b''
                 are the posts we replied to.
             -->
             <previous>
-                <id>id1/3</id>
-                <id>id23/5</id>
+                <id key="id1" local-id="3" />
+                <id key="id23" local-id="5" />
             </previous>
             <next>
-                <id>id2/3</id>
-                <id>id43/5</id>
+                <id key="id2" local-id="3" />
+                <id key="id43" local-id="5" />
             </next>
         </model>
         <model name="post">
-            <id>id1/2</id>
+            <id key="id1" local-id="id2" />
             <title>13</title>
             <text>Thirteen</text>
             <pub-time>12</pub-time>

docs/samples/response_pull.xml

@@ -2,9 +2,9 b''
 <response>
     <status>success</status>
     <models>
-        <id>id1/1</id>
-        <id>id1/2</id>
-        <id>id2/1</id>
-        <id>id2/5</id>
+        <id key="id1" local-id="1" />
+        <id key="id1" local-id="2" />
+        <id key="id2" local-id="1" />
+        <id key="id2" local-id="5" />
     </models>
 </response>