Improve look and feel of default pdf export

The current default pdf export is as raw as it gets and could be slightly improved with a couple of easy changes:

• export with a sans serif font instead of a serif font - this is a matter of preference, but I personally find the sans serif more elegant. It would also match the type of font used by the XWiki web UI, even if it wouldn't use the exact same sans serif font
• make a clearer separation between the header and footer and the page content
• add text on the cover page explaining the username that gets displayed in there and the date
• the page header displays the raw name of the space of the exported page, and this is no longer the practice in XWiki, it should display the display title of that page