Fix of https://github.com/prusa3d/Slic3r/issues/285

Refactored Model.cpp/hpp to C++x11 loops,
simplified the mesh / bbox handling.

utils/wireframe.pl

@@ -38,7 +38,6 @@ my %opt = (
 {
     # load model
     my $model = Slic3r::Model->read_from_file($ARGV[0]);
-    $model->add_default_instances;
     $model->center_instances_around_point(Slic3r::Pointf->new(100,100));
     my $mesh = $model->mesh;
     $mesh->translate(0, 0, -$mesh->bounding_box->z_min);