Fixed problem with incorrect modal window position

According to [comment](https://github.com/samdark/the-modal/commit/f877e572982246c88da8a9335a9141824ec1387b#commitcomment-6656592) from @SukhikhN there is was problem with incorrect margin value calculation. 

Additional tests shows that problem also arises in a case of non-zero margin values for ```html``` and/or ```body``` tags. This patch fixes both problems. Tested in latest Firefox, Chrome and IE9+.