Updated equella_guess_icon to allow 24x24 icon size in Moodle 2.4 plus a...

...dded new images to pix and added version required to 2.4

Also closes this issue I raised last week
https://github.com/equella/moodle-module/issues/7

I have made a tool as well to rebuild the course cache to show the new icons, currently pending approval to Moodle Plugins directory

https://github.com/agrowe/moodle-tool_rebuildcoursecache