Switched react mixin to use setState instead of forceUpdate

from the React docs..
>NEVER mutate this.state directly, as calling setState() afterwards may replace the mutation you made. Treat this.state as if it were immutable.

This was bugging me a bit, let me know if you see any issues with my implementation. I suppose this is slightly less efficient, as every store's `getState` method will be called on each change event, but it would take many very large stores to create any noticeable performance hit.

Another solution would be to put each `storeName` as a key directly on `state`, but this of course creates a higher probably of a name conflict with a `state` the developer has applied to the view themselves.