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
Never throw an error. 

You may do it in some special cases and if you want the process to terminate / to prevent startup / etc., but *never* do that as a reaction to user output.

You don't have usual exception handing chain. *Always* `cb err`. 

To distinguish between variety of errors, either subclass `Error` or assign a significant attribute.