[RFR] Add "Download JSON report" link

For embedding the chart of a given report without recomputing it each time, we can use the transformer JSON report directly, without calling the `ReportTransformer.transform` method. Thanks to the JavaScript `Blob` object, we can force the download of the file to the end-user directly.

![download-report](https://f.cloud.github.com/assets/688373/1895783/cec6db8a-7b5d-11e3-823c-530e52735fd6.gif)