add support for browsers

It's unfortunate that this diff looks so messy; it's all just from indenting the code after wrapping it in a function.

Basically, this change wraps the code like this:

```javascript
(function(exports) {

  // ... existing code, untouched

}(typeof exports !== 'undefined' ? exports : (window.sass = {}));
```

So, in the event `exports` is defined, the behavior is unchanged. Otherwise (i.e., in a browser), all of the functionality that would have been exported in Node gets assigned to `window.sass`.

This addresses visionmedia/sass.js#20.