Flip element when it's attached to target's center with `together` attachment

When element is attached vertically and/or horizontally to target's center through `together` attachment, the element's attachment stops *flipping*. [This fiddle](http://jsfiddle.net/TuFnv/embedded/result/) illustrates the issue (leave container through its right edge).

I believe this is an unexpected behaviour, which is fixed by this pull request.