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.
Why not 0? You can drop `else` with it:

```
var total = files.length;
var current = 0;

current++;
if (current === total) {
  resolve();
}
```