Playground cards use relative paths.
Fix relative path loading.

Previously relative paths for nested cards would have an effective base URI of
the topmost card.

This also expands `Conductor.MultiplexService` to support optional data
transformations for requests and events.  For example, the following is used to
transform requests in child cards so that they become absolute URLs immediately,
after which they are propagated to the containing environment and requested
directly.

```js
  this.conductor.services.xhr = Conductor.MultiplexService.extend({
    upstream: this.consumers.xhr,
    transformRequest: function (requestEventName, data) {
      var base = this.sandbox.options.url;
      if (requestEventName === 'get') {
        data.args = data.args.map(function (resourceUrl) {
          var url = PathUtils.cardResourceUrl(base, resourceUrl);
          return PathUtils.cardResourceUrl(document.baseURI, url);
        });
      }

      return data;
    }
  });
```
Merge pull request #11 from tildeio/djh/relative-resources

Fix relative path loading.