Allow defining store mixins via a spec syntax similar to Component mixins.

Fixes #32.

I found that I was defining similar functionality on a number of stores, especially if they listened to the same data.

Similar to React components, this will merge actions, chain functions, and attempt to merge objects/arrays. It will not, however, merge the result of functions (like [createMergedResultFunction](https://github.com/facebook/react/blob/ef6740627292136973a6b8053cc98dba98ebd18f/src/core/ReactCompositeComponent.js#L600)) as we have no such use case.

All the mixin functions are assigned directly to `createStore` so they can be overridden by users if necessary; for instance, I could see users wanting to define a different `merge` policy, such as deep merge.