Updated make script + bug fix

Hi,

after a few minor modifications (and compiling with the latest version of emscripten) I was able to get toFastBitmap to work. Here's a code snippet from a local test:

```javascript
{
	var reader = new FileReader();
	reader.onload = function(e) {
		console.log('Converting to image, please wait');
		var img = new Image();
		var rawson = new Rawson.File(file.name, e.target.result);

		img.src = rawson.toFastBitmap();//rawson.extractThumbnail();

		img.onload = function() {
			console.log(img.width+'x'+img.height);
			cb(null, img);
		};
	};
	console.log('Loading... Please wait.');
	reader.readAsBinaryString(file);
}
```