Added a Download-Button for Images

! The changes were made in the compiled files *.js *.css and not in the
source files *.coffee, *.sass !

Next to the Close-Button there is now a Download Button which redirects
to a PHP Script, e.g. download.php?filename=images/my/image.jpg
The Script Path can set in LightboxOptions.
The Download Button is very ugly and needs a remake.