fixed calculation and checked how the script works with png with the same amount of pixels for all colors used on the image
removed gm module, did small refactoring
removed node-canvas module and use get-pixels instead
moved getBgColorForImage.js to /lib and made it a module reused in jobProcessors.js
small refactoring - two loops instead of 4
update status and background color in one query
use tile_set.background_color while rendering a map