Avoid calling setState when component is being unmounted

[mridgway]

This avoids an issue where you have a parent and a child both listening to the same store, where parent could re-render in the middle of handling the change event causing the child to be unmounted. When this happens, the child's call to setState will cause a warning.

It's important to note here that the isMounted() may be removed in the future: https://facebook.github.io/react/docs/component-api.html#ismounted. If this happens we will either need to implement our own flag (using componentWillUnmount) or hope that an alternative is found. See facebook/react#2787 for discussion on this warning and whether it's needed or not.

Also fixes YahooArchive/fluxible-router#18

[yahoocla]

CLA is valid!

[jedireza]

👍

[geekyme]

Thanks!