Add UMD support

Should fix #60

The diff seems pretty messy because of the re-indent. You should merge this one in very last.
To sum it up, all I did was wrapping Sketch around this.

```
(function (root, factory) {
    if (typeof exports === 'object') {

        // CommonJS like
        module.exports = factory(root, root.document);

    } else if (typeof define === 'function' && define.amd) {

        // AMD
        define(function() { return factory(root, root.document); });

    } else {

        // Browser global
        root.Sketch = factory(root, root.document);

    }

}(this, function (window, document) {



   // old Sketch go re-indented here


    return Sketch;

}));
```