add a contribute.json file (bug 1074452)

I did my best to find all the data to populate the json file, but l'm assuming I'll need to make a change or two. 
The point is not to be human readable, but data for automated tools to display contribution information. I'd worry that `text/plain` would confuse the intended audience. Also you can just tell your browser to display JSON if you want, or get an [http://jsonview.com/](addon that makes looking at JSON nice).