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
not really sure..

if your option is for example :
```javascript
options : {
   spritePath:'test/sprite/sprite.svg',
   cssPath:'test/css'
}
```

with "test/css" as a folder path.dirname() will remove "css" in cssPath (see : [node.js api](http://nodejs.org/api/path.html#path_path_dirname_p) ) and we will have in 'test/css/[task]-sprite.css' :

```javascript
background-image: url("sprite/sprite.svg")
```
instead of
```javascript
background-image: url("../sprite/sprite.svg")
```

i think we need to keep test if filename is in cssPAth with "." indexOf.
Are you agree ? 

maybe i can write a inline if version for shorter code if you want.