Add JSON index page.

My go at fixing [bug 962732](https://bugzilla.mozilla.org/show_bug.cgi?id=962732).
Adds a `/json-snippets/` page that lists all the JSON snippets with the relevant info (name, text, url and icon). This is basically taking the snippets page and reusing it with some minor modifications and looks like this:
![screen shot 2014-11-05 at 14 33 08](https://cloud.githubusercontent.com/assets/318208/4927598/073a8eac-653d-11e4-8880-a5518f3b0734.png).

I try to keep PEP8-ing to a minimum when making changes so that a PR is to the point as much as possible but these changes snuck in :)