Convert relative urls to absolute url when merging CSS files
Add tests for the CSS url rewriting
Add `minifiers-tests.js` into package.js` (tests were not executed)
Handle the case where there are more than one url in the same value
Merge pull request #1986 from mquandalle/css-urls

Convert relative urls to absolute url when merging CSS files

Fixes issues introduced in `css-minification` branch in development mode and caused by concatenation in general.

When a stylesheet refers to an asset by a relative url, the relative url becomes invalid after concatenation if the stylesheet was originally located somewhere nested.