Update API documentation.
Add removeOverlay() and ability to force render.

Add the removeOverlay() method.
Add the partiallyUninterprete() method.
Add the $force second argument of the render() method.

So now, we are able to do such thing:

    $xyl->addOverlay('Overlay1.xyl');
    $xyl->render();

    $xyl->removeOverlay('Overlay1.xyl');
    $xyl->addOverlay('Overlay2.xyl');
    $xyl->render();

\o/