ie8: tooltip

domUtil will use jQuery to compute position and offset if jQuery is present.
jQuery can then be a requirement for react-bootstrap + ie8.

I know even mentioning jQuery in the source will be unpopular,
but it seems like the most reasonable solution.
It seems rare that `$` would a jQuery instance but `jQuery` would not be present; it seems less rare that `$` would point to something that's not jQuery. Why not just check for `window.jQuery` here?