Improve handling of absolute paths

[dagostinelli]

Given an HTML file containing a css resource using an aboslute path: <link rel=stylesheet href=/css/site.css>

Result:

$ htmlark mypage.html
**SS: Error reading '/css/site.css': No such file or directory
Unable to convert webpage: [Errno 2] No such file or directory: '/css/site.css'**

I think htmlark should be able to handle this better.