Fixed issues in calculate-position.js:
Fixed issues in calculate-position.js:

- Added support for when the page is horizontally scrolled. The function wasn't taking
  `scroll.left` into account, which causes the dropdown to drift to the
left as you scroll right.
- Improved horizontalPosition=auto to work better for dropdowns that are
  smaller than their trigger element. It now considers which position
will show the most horizontal pixels of the dropdown in either left- or
right-aligned mode and updates the position accordingly.
- Made sure that `style.left` is set to `null` when `style.right` gets
  set. The dropdown element should only have a `left` or a `right` css
property at any time.
- Reorganized the algorithm a little. No need to set `dropdownLeft` fx when
  we end up setting `style.right`. Also added some comments about the
parts that are less obvious/hard to grok.

cibernox/ember-basic-dropdown,cibernox/ember-basic-dropdown,cibernox/ember-basic-dropdown,cibernox/ember-basic-dropdown