Minified CSS file not copied in target/www/

(aka : Who forgot to propagate #15 to CSS  ? me !)

Minified CSS artifact was not added to www folder (and thus not in .zip)

* Changed minification output file to target/www/artifact-min.css (as aggregated file)

* After minification, copy target/www/artifact-min.css to target/artifact-min.css

Signed-off-by: nicolas-rempulski <nicolas.rempulski@gmail.com>