ie8: update test cases + readme

* popover test case
* accordian test case (failing)
Only pass through onSelect handler if it has been set on the DropdownButton
or it's MenuItem child

Fixes #138
Merge pull request #139 from pieterv/dropdownbutton-onselect-fix

Only pass through onSelect handler if it has been set on the DropdownButton
Fix infinite render loop in `CollapsableMixin` in non-CSS
transition supporting browsers (ie: ie8/9)
Fix `Carousel` examples for ie8
Merge pull request #137 from react-bootstrap/ie8-accordion

Ie8 accordion
Merge pull request #140 from react-bootstrap/ie8-carousel