Memory leak with observer

[jsaunderson]

There is a memory leak when using an observer, this appears to be caused by mixinLifecycleEvents not patching componentWillUnmount resulting in the reaction never being disposed.

After adding patch(target, 'componentWillUnmount'); the memory leak went away.

Is there any reason it was omitted, or can it be added in?

[mweststrate]

@jsaunderson Looks like that is missing indeed! Feel free to open PR