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
Hah, now I see it. Looking at these several lines of code (reading stderr), it should be refactored - the code was altered several times and I see there are leftovers. It should be handled the same way as `stdout`.

Used `.trim()` because `stderr` ends with `\n` naturally, but I wanted to use it in error message, where it looked silly.