Added: url to Attachment#to_json refs #108

I added ```url``` field to attachment json.

If you use ```LocalStorePolicy```, you'll get json like below.

    attachment: [
        {
            disk_filename: "d1b8ed40-ee95-40c2-8598-56f7b5c73e12-test.jpg",
            filename: "test.jpg",
            content_type: "image/jpeg",
            url: "/upload/d1b8ed40-ee95-40c2-8598-56f7b5c73e12-test.jpg"
        }
    ],

If you use ```WatagePolicy```, you'll get json like below.

    attachment: [
        {
            disk_filename: "",
            filename: "test.jpg",
            content_type: "image/jpeg",
            url: "http://dl.dropbox.com/u/xxxxx/5af9ac23-749f-4128-b6bd-69386d4b4670_test.jpg"
        }
    ],

This means AsakusaSatellite newly provides an easy way to get permalink of attachment files.