#45 Fix ordering bug with add function on WebKit

The function `htmlDoc` need to keep the order in `elems`.
But in JQuery, there's a bug with `add` and the order. (https://github.com/jquery/api.jquery.com/issues/254 or http://bugs.jquery.com/ticket/13331)

A solution is to use `append`. (https://github.com/vakata/jstree/commit/9c41e435d5aee9647e26500200e30b359bb96ae0)
Can't work for me.

So I imagine this solution with `merge`. I don't know if it's the best, but it seems to work !