Change types of some methods in Document and Screen

- Method Document#elementFromPoint(x, y) takes arguments of type Double
   http://www.w3.org/TR/cssom-view/#extensions-to-the-document-interface
- Width and height properties of Screen has type Double
   http://www.w3.org/TR/cssom-view/#the-screen-interface
Merge pull request #58 from andrey-fomin/types-in-document-and-screen

Change types of some methods in Document and Screen