On widows file paths will include \ instead of / and source-map doesn’t convert slashes, but source map will work only with UNIX slashes.

We can fix it in app, but I think, that we need to fix it here, because every source-map node.js user must to repeat this fix.

Autoprefixer issue: nDmitry/grunt-autoprefixer#25

I can helps with PR, but how we should do this? Try to load path (it wil be in node.js and browserfy) and if path.sep == "\\" replace all \ to /?