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
This *is* a reaction to user output; you are executing this code because they requested the composition. 

The error condition doesn't happened because of input but because of `matter_compiler`, yes...and IMHO, your reaction is going to cause it to hang and time out. Test it. 

Code to study:

```
spawn = require('child_process').spawn
try:
  ls = spawn 'ls'
  ls.stdout.on 'data', ->
    throw new Error 'You think you can change that?'
catch err
  console.error 'I wonder what is going to happen', err
```