Optional watchStores component property

In larger applicaitons, it may become inefficient to watch all stores for changes. The `watchStores` property is an array, defined on your component, containing the names of stores you want the component to watch for changes. This property is optional, and when omitted, all stores associated with the passed `dispatcher` will be watched. Store name strings should match the keys of the stores returned in the `dispatchers`'s `getStores` method.

There are other way we could handle this functionality, like passing parameters to the mixin, but that would require breaking changes, as the mix would need to be a `function`, which return an object.