* Add `react-overlays/Overlay` component test.
* Fix `react-overlays/Overlay` and `react-overlays/Modal` components.
* Remove `react-overlays/Transition`.
* Set `react-overlays` version to v0.8.
* refactor react-overlays into individual modules
also adds correct definitions for some modules whose definitions
were incorretly being imported from react-bootstrap.
(react-bootstrap depends on react-overlays, not the other way around)
* use exported namespace hack instead of default exports