Clarify which release is covered by install instructions

Thanks Philippe Clérié for suggesting to explicitly mention wheezy
Clarify ftdi_sio parameters are only needed on really old kernels

Thanks Philippe Clérié
Fix broken links to plugcomputer.org
Fix internal links
Fix broken external links
Stop linking to the obsolete photo gallery
Fix version number when discussing upgrades from Debian 6.0

Thanks to Hugo Ricketts for pointing out the mistake.
Add a note about the need to upgrade to u-boot 2011.12-3 (or higher)

Thanks to Bob Smeets for suggesting that the note on my blog is not
enough.