Base the CSS fileUrl always on the baseUrl

The fileUrl is based correctly if the module ID is a relative path.
If the module ID starts with a prefix defined in the configuration,
RequireJS resolves the module path to the input root instead of the
output one.

if it comes from the source
+    // rebase to the output directory if based on the source directory;
+    // baseUrl points always to the ouptut directory, fileUrl only if
+    // it is not prefixed by a computed path (relative too)
Fix typo in comment
Merge pull request #158 from prantlf/fix-url-rebase

Fix the file url to be based always in the output directory