Added support for HTTP authentication to the WeasyPrint application

I ran into the problem of having to access a document that is only available to authenticated users. Authentication options are missing in the WeasyPrint application, which this patch fixes.

The patch has been tested on Python 2.7 and 3.2. A testcase has been included that checks both basic and digest authentication schemes.

Again I'm happy to fix any issues you might have with the patch.

Cheers,
Colin