compileAllJsFilesInDir: Added promise

If I should execute other scripts after traceur compilation I need to know when compilation is finished.
But seems in this PR one problem - on reject way.

errback at `.compileSingleFile` - only if there are problems in file reading. Parse errors are [thrown](https://github.com/tuchk4/traceur-compiler/blob/master/src/Compiler.js#L170)  in other scope.
This should then be:

```js
if (err) {
  reject(new Error('While scanning ' + inputDir + ': ' + err))
  return;
}
```

But we also need to terminate the `glob`.