Issue title: Minifying garbles certain css expressions
 Issue body: I have the following in my stylesheet

``` css
*color:expression((new Function('elem', "elem.style.color = elem.parentNode.currentStyle['color']"))(this) );
```

which after processing becomes

``` css
*color:expression(22230);
```

 Comments: 
 Comment 0: It's not the minification that causes this, the processing interprets the unnamed parens as calls to the math function.

Will look into adding the math function escaping.

 Comment 1: Ah! Makes sense that its not the minification itself. Thanks for your work.

 Comment 2: Fixed in master branch.

Putting a tilde in front of a property name will escape a declaration

{'source': 'GithubIssues', 'repo_name_with_owner': 'peteboere/css-crush', 'issue_id': 3707606, 'repo_stars': 535.0}