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. 
If you do it like this https://github.com/mozilla/airmozilla/blob/master/airmozilla/urls.py#L14-L15 you get the benefit that it works without nginx and it doesn't require a django view. It also makes it easy to add a rule in nginx to serve it straight from there and you don't have to worry about caching.