Support wildcarded export

support this valid ES6 module syntax:

```
third_party/javascript/angular/v2_alpha/v2_0_0_alpha_12/angular2-es6/angular2.es6:4: ERROR - ES6-to-ES3 conversion of 'Wildcard export' is not yet implemented.
export * from './forms';
^
```

Also strange the command line options were:

```
... '--language_in=ES6' '--language_out=ES5' ...
```

But the error message mentions 'ES6-to-ES3 conversion' - not sure what that's all about.