polsy.org.uk-ify video site links

Converts page links to several popular video sites (including YouTube and Nico Nico Douga) to instead link to an embed page on polsy.org.uk which displays the video centered on a mainly blank page, and resized to a usually more appropriate resolution (ie one matching the resolution of the source flash video, to prevent visual/CPU issues caused by resizing). The sites enabled can be configured from the extension's Options page at chrome://extensions YouTube defaults to off as it's the most intrusive Links on the video sites themselves will never be modified. Version 3 update: - remove obsolete video sites - fix timestamps on youtu.be links