Update Topmenu.php to fix issue #24

Additionally removes dropdown-carat on toplinks with no children.
Fix wrong DropdownLink

When using toplinks with no children and toplinks with children, and you set catalog/navigation/top_in_dropdown to Yes, you would end up with the wrong top-links.

In my case I had 3 toplinks with no children and one 1 toplink with children and the top_in_dropdown link was the 1st toplink instead of the 4th. 

I fixed this by adding a check for class="parent" in the regex for _addDropdownLink();
Update Topmenu.php / Small RegEx fix

Damn you RegEx! :)
Merge pull request #28 from ottonet/Dropdown-Fix

Update Topmenu.php to fix issue #24