Add compress() method to Document

[slusarz]

For maximum compressibility (i.e. removing all unneeded whitespace and semicolons), might want to add this method to Document:

    public function compress()
    {
        return str_replace(
            array("\n", ' {', ': ', ';}', ', '),
            array('', '{', ':', '}', ','),
            strval($this)
        );
    }

We currently have this in the wrapper object around the Document object, and it seems to work fine on all browsers. At the least, it outputs code that looks very similar to what jquerymobile does to compress their CSS (i.e. taking their CSS -> importing into PHP-CSS-Parser -> strval($document) -> compressing output = output almost identical to the input).

[sabberworm]

This is a duplicate of issue #6 (which, believe it or not) is in the works.