PDF page rotation

Hi,
first I would like to thank you for a great library.
I really like the balance between friendliness and efficiency you have chosen here.

I am implementing a simple tool for appending and possibly merging pages from various PDF sources.
Formerly I was using the `pyPdf` library (http://pybrary.net/pyPdf/) and decided to switch to `PDFWriter`.

Unfortunately I did not find a high level support for one operation I need - page level rotation.
The `Rotate` attribute seems to me very similar to `MediaBox` and so I tried to extend the `PDFPage` and `PDFPageInput` to provide high level support for it.

I created a new test for it and everything seems to work as expected.

I hope you will not oppose adding it to the library, feel free to ask me for any adjustments you'll find appropriate.
Or, if you find it inappropriate overall then it is OK for me to hold my changes in a separate branch. 

Kind regards,
Petr Filipsky