PDF generation

[tim-heinsohn]

Let the bundle allow invoiced to be rendered as PDF (as one option beneath HTML).

Step 1: via the existing web interface

Step 2: as Console task

[quafzi]

What do you think of rendering Restructured Text instead of HTML by default. So we had only one template to maintain, and all the different output formats can be handeled by rst2pdf, rst2html, ...