MINOR: stats: Add JSON export from the stats page

It is now possible to export stats using the JSON format from the HTTP stats
page. Like for the CSV export, to export stats in JSON, you must add the option
";json" on the stats URL. It is also possible to dump the JSON schema with the
option ";json-schema". Corresponding Links have been added on the HTML page.

This patch fixes the issue #263.
3 files changed