fix(tooltip): update placement on content change

Given that the recompute of position is a heavy calculation and the only immune placement to this bug was `right` ,
two kind of fixes had to been made:
1. For `left` placement use the right css style option instead of left.
2. For the `top` and `bottom` placement keep reference of the last known middle position of the directive element
  and use it when re-calculating the position with the new tooltip width.