Added setPaperSize functionality

Thank you for this very clean and clear node-plug-in.  I found support for `setViewport`, but no support for `setPaperSize`, which is necessary for rendering PDFs with PhantomJS.

I added `setPaperSize(obj)`, where `obj` is identical to `page.paperSize` described at http://phantomjs.org/api/webpage/property/paper-size.html.