ZIP download put a slash linking to assets and css

[FablabMurat]

ISSUE

When you publish your website as ZIP (download the static content), the assets and css are absolute path ("/assets/img.jpg" and "/css/mystyle.css").
Unzipping on a folder display the web site broken .

PROPOSITION

Make relative the path ("./assets/" or "assets/")

[lexoyo]

Hello
as you're not the first to ask this I think we should do it
Would you be able to make a PR?
This is where it should be done
https://github.com/silexlabs/silex-lib