submissions.json: Add JSON-P support

* `/submissions.json` would return `[{…}]`.
* `/submissions.json?callback=foo` would return `foo([{…}])`.
Merge pull request #24 from mathiasbynens/master

submissions.json: Add JSON-P support
Catching repo .htaccess up to changes from #20
Merge branch 'convert-p-classes' of github.com:mezzoblue/csszengarden.com into convert-p-classes