Exclude diff.js from build as it pollutes global namespace.

This one cost me a few hours ;)

Summary: `/angular-diff.js` included `/src/diff.js`, which includes this function:

```javascript
function escape(s) {
    var n = s;
    n = n.replace(/&/g, "&amp;");
    n = n.replace(/</g, "&lt;");
    n = n.replace(/>/g, "&gt;");
    n = n.replace(/"/g, "&quot;");

    return n;
}
```

Thereby overriding the native JS `escape` function. Facebook JS SDK uses this function, and all kinds of chaos ensue.

This PR excludes `/src/diff.js` from concatenation and minification.

Thanks for your work on this package.
Caden