Fixed Panels to update height when contents change.

This addresses #207 - CollapsableMixin now sets the dimension style attribute to 'auto' when the element state has finished changing from collapsed to expanded.
It looks like its set from in the components that use the mixin, https://github.com/react-bootstrap/react-bootstrap/blob/master/src/Panel.jsx#L35, but yes sorry agree its all a little odd, im not familiar with this code.