create mixin for simple `change` communication between ComponentView and Store(s)

[0xgeert]

A Mixin called SimpleChangeControllerViewMixin implementing the following

With this mixin the Controller-view <--> Store communication is abstracted away as such: *

• - A function getStoreState is needed on the Controller view
• - A prop stores (single obj or array) is needed on the Controller view
• - A function _onChange is needed on the Controller view *
• - This getStoreState is called on getInitialState
• - getStoreState may contain properties from every store in this.stores
• - the Controller-View registers a ChangeListener on every store defined in this.stores
• - this ChangeListener does a setState(getStoreState()) *
• This is useful for stores to automatically register ChangeListener events.
• NOTE: if a store implements other notifications besides 'change' these may be registered in addition
• by implementing addXListnerner / removeXListener in componentDidMount and componentWillUnmount respectively.