Affix relative to parent - with automatic offsetBottom calculation.

I also changed the affix variable to be 'middle' instead of false (javascript does not care about type-safety, but still...).
Also, in top/bottom mode the position is not necessarily 'relative'. It can be 'absolute' inside a 'relative' (see example.html).
The behaviour in the "parent-mode" is a little bit different than the behaviour in the original mode,
but I think it is now correct.