Implemented template support for 'rel' property on href's schema

[perevernihata]

If you're trying to use template feature with 'rel' property in "links" object (like on the json bellow), it does not work in the vanilla editor.
Here the direct link to an example - http://goo.gl/Q135mw

{  
   "type":"array",
   "title":"Dynamic urls",
   "items":{  
      "type":"object",
      "id":"related_im_prop_item",
      "properties":{  
         "name":{  
            "type":"string",
            "watch":{  
               "link_id":"related_im_prop_item.id",
               "link_title":"related_im_prop_item.name"
            },
            "links":[  
               {  
                  "href":"/test/path/{{link_id}}",
                  "rel":"{{link_title}}"
               }
            ]
         },
         "id":{  
            "type":"string"
         }
      }
   }
}

This change is aimed to fix the problem.

[perevernihata]

Well, basically, I've found a little bug in my code. It throws an exception in case 'rel' is not defined. Am I supposed to add another pull request, or should I just post one-line hotfix here?