Prevent an extra "../" in background-image url with cssPath filename option

if you specify a filename in options.cssPath, the path.relative function return an extra "../" level due to the filename.
Add a test if contains "." return cssPath dirname
Good catch!

It could also just be:
```js
var dirname = path.dirname(config.cssPath);
relation = (dirname != config.cssPath) ? dirname : config.cssPath;
```