Page orientation

In regards to [Issue 498](https://github.com/dompdf/dompdf/issues/498), and with the code I posted on Google Groups:

https://groups.google.com/d/msg/dompdf/03akDzpb1mo/EsZQC0ain6AJ

I did notice an issue with Adobe Reader, whereby it wanted a pages->pages->page type structure.

I've fixed this and thought I'd have a go at a pull request.

In summary, this has resulted in some changes needing to be made to cpdf (for the 'pages' object to have a parent, and to count the children), and I've exposed the "new_document()" method via the dompdf class.

There is also an example file, which I've just dumped into a /testing/ folder for now.

Hope you find this useful.