Issue title: Corner stamp failes in <IE9 when no maring-left specified in CSS
 Issue body: When no maring-left specified in <IE9 it code $cornerStamp.css('marginLeft') in the example returns NaN, which leads to expression 

cornerStampX = $cornerStamp.offset().left -
                        ( this.element.offset().left + this.offset.x + parseInt($cornerStamp.css('marginLeft')) );

be NaN too. You have specify margin-left in the CSS for corner stamp div, or change JS.

 Comments: 
 Comment 0: tomkiv, thanks a lot!
I couldn’t figured out why it was not working on IE.

Very useful.

 Comment 1: [Masonry v3 was just released](http://masonry.desandro.com). It has much better support for stamps. Please give it a try. If you continue to experience problems, report back and I'll re-open this issue.

{'source': 'GithubIssues', 'repo_name_with_owner': 'desandro/masonry', 'issue_id': 2030275, 'repo_stars': 15816.0}