Rename the README until it is checked and confirmed it actually works like that

It seems to be pretty outdated now.
Update the geshi lib to 1.0.8.11

The main motivation for this upgrade was MDLSITE-2693 that is addressed (among
other things) in this release.
Do not use CSS classes for actual styling any more

Sorry, we don't have time to maintain a CSS for this for moodle.org purposes. I
believe that plain hardcoded values are good enough for now.
Add missing version.php and call it GeSHi v1.0.8.11-r1

The first part of the release is the geshi lib version the filter is currently
using. The second part is our own revision number.