twitter-compatible API: /friendships/exists.json

[carols10cents]

This is a smaller story broken out from #562.

This issue is just to make /friendships/exists.json work like the twitter API. This route doesn't currently exist at all, but there is a nice update json decorator for you to use.

Ignore the issue of client authentication for the moment; just make this work as a normally authenticated user in the browser for now (see #564 about authentication)

https://dev.twitter.com/docs/api/1/get/friendships/exists

Description: Test for the existence of friendship between two users. Will return true if user_a follows user_b, otherwise will return false.

Definitely interpret anything in the twitter docs so that it makes sense for rstat.us, and definitely ask if you have any questions!

Leave a comment if you're working on this.

[Nakort]

Working on this