FileSourceDGS with ES6 promises

The FileSourceDGS class extends the FileSource generic class. It handles
the asynchroneous file loading with a Promise. Promises are ES6
standards. So the library now depends on a shime.

The New way to deal with FileSourceDgs is as follows:

```javascript
var graph = new GS.Graph('#canvas', 'svg');
var source = new GS.FileSourceDGS();
source.addSink(graph);
var promise = source.begin('../../test/graph.dgs');
promise.then(function(source){
	// the file has been read. Now we can parse it.
	while(source.nextStep()){
		//read the dgs file, step by step
	}
} , function(err){
	throw err
});
```