Tag API: Primary Document Format

Closes #2605

- Change tags browse() response to { tags: [...] }
- Update client side collection to use nested tags document
- Update test references to use response.tags

ghost/admin/models/tag.js

@@ -3,6 +3,10 @@
     'use strict';
 
     Ghost.Collections.Tags = Ghost.ProgressCollection.extend({
-        url: Ghost.paths.apiRoot + '/tags/'
+        url: Ghost.paths.apiRoot + '/tags/',
+
+        parse: function (resp) {
+            return resp.tags;
+        }
     });
 }());