set custom CORS header in tilemill

cors.diff

diff --git a/servers/Tile.bones b/servers/Tile.bones
index c80ac44..07b7243 100644
--- a/servers/Tile.bones
+++ b/servers/Tile.bones
@@ -65,6 +65,7 @@ server.prototype.getArtifact = function(req, res, next) {
             if (err) return next(new Error.HTTP(err.message, 404));
             if (res.cache) fs.writeFile(res.cache, tile);
             if (headers) headers['max-age'] = 3600;
+            headers['Access-Control-Allow-Origin'] = '*';
             res.send(tile, headers);
         });
     });