v2.0.2

I ran into some trouble adding the slider to a project, so here comes the first wave of bug fixes:

* `_dragEnd` now removes the mouse event listeners. This had a serious performance impact and was just a really bad mistake on my part.
* `onChanged` is now being called correctly
* marked React as a dev dependency, so it won't be installed if you add the slider to another project via `npm install react-slider`. This was causing problems with browserfy.

Hope you can merge the fixes soon.