change relative-url detection method

When a css file contains relative resource like `url(a.png)`,
the library cannot transform it because the file is recognized
as an absolute url. But actually it is a relative url.

So I change the detection to:
    
1. Urls starting with slash (`/`) is considered absolute.
2. Urls starting with a word and following with colon (`:`) such as `http:`, `https:`, `file:` is considered absolute.