Provide more options for PDF Export handling of header/footer

Problems:
1. The PDF document has the same header and footer on all its pages, instead of having the header and footer from the XWiki page coresponding to the current PDF page.
2. No matter what you specify in the pages variable in the URL, the header/footer correspond to the page where the export process has been triggered from, even if that page is not given trough the pages variable.

How to reproduce:
(1)
http://playground.xwiki.org/xwiki/bin/export/XWiki/Admin?format=pdf&pages=Main.WebHome&pages=XWiki.Admin
http://playground.xwiki.org/xwiki/bin/export/XWiki/Admin?format=pdf&pages=XWiki.Admin&pages=Main.WebHome
(2)
http://playground.xwiki.org/xwiki/bin/export/XWiki/Admin?format=pdf&pages=Main.WebHome