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
Isn't this a special case? I thought it's the same as if the error was right in the coffeescript code. The same would happen (uncaught exception). Only it's in Ruby subprocess, not in the app itself. However, the reason is the same - unexpected error we're not able to handle. If `matter_compiler` fails badly, failure & papertrail & HTTP 500 should follow. Or it shouldn't?