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 pull request could fix the issue.
Merge pull request #10 from eskimoblood/patch-2

Use baseDir also for relative paths