Use baseDir also for relative paths

I use the grunt-usemin task to concate the css files from html page. 
So now I have all my files in a single css file my dist folder, using your task I cant find the images as I can't set the root for my relative image paths.
This change fix the issue for me.

Btw. my image path starts with `../` maybe thats an issue as you only check for `/`.