Add API blueprint documentation

I've added the boxychat.api file containing an API Blueprint compatible definition.
I've opted for API Blueprint "style" because it give us some advantages, for example we could use the following tools.
**Aglio** is an API Blueprint renderer with theme support that outputs static HTML. We could use it to generate our documentation, we can even have themes, so it's up to our taste.

**Dredd** is an API Blueprint testing tool. This one is quite cool, with always the same file of documentation we can give it to Dredd and it is going to run it against our APIs. We could integrate it in Travis CI

Here's an example of how it looks with Aglio: http://cl.ly/image/0p3a3m3N0b36