Beginnings of making Slot#top work

* unslot the Slot (e.g. absolute positioning) if left or top are
  supplied
* refactored so that current_positiong starts straight with
  margin_left and margin_top so they don't have to be added in
* removed explicit top/left from top slot (not needed)
Call contents_alignment once evaluing the block is finished

* then elements are already correctly positioned
* the unslot approach is not totally right (refer to #217)
  but better than what we have so far/a step in the right direction
Add the left margin when moving to a new line

* fixes #164
fix test failure
Some margin specs for slots