Blueprint composer

https://trello.com/c/WZWswaln/1740-api-blueprint-api-composer

Related: https://github.com/apiaryio/matter_compiler/issues/5
Before release:

```bash
$ heroku config:set BUILDPACK_URL=https://github.com/ddollar/heroku-buildpack-multi.git
```

After merge: PLEASE, reopen https://github.com/apiaryio/apiblueprintorg/pull/13
Hmm, actually, showcrash/protagonist doesn't send the errors when blueprint is invalid? 

Because I think we should propagate those to the clients as well...