Optimise image delivery.
Merge pull request #92 from gjb2048/master

Optimise image delivery.Very nicely done. A far more elegant solution than my own. Thanks for sharing. My shortcomings in PHP do show occasionally :)
Adding issing lang strings
reoving outdated grader styling
New color styling for gradebook
adding new stylesheet
adding ability to set custom category icons
minor change to z-index of custom menu
Fixing custom icons on manage courses screen
Fix for indenting of categories