Store mixins

Just implemented very simple version.
Need feedback about spec I intend to implement.
Especially about `init`, `preEmit` and `shouldEmit` methods.
I think, it'll be cool to add possibility to mixin multiple init methods. Some mixins can require `init` for it's needs, but let user to write another `init` in store defenition. Same about `preEmit` and `shouldEmit`

Also updated Grunt version in package.json. Only with `0.4.0` `npm install` works for me. Otherwise it errors about Grunt version
```
The package grunt does not satisfy its siblings' peerDependencies requirements!
```
Not sure is this fix is ok.
Instead of this maybe create smth like in React
https://github.com/facebook/react/blob/master/src%2Fclass%2FReactClass.js#L33-L53
With specifying which methods can be defined several times, and how to merge them.
For `init` smth like `DEFINE_MANY`. For `preEmit` use `DEFINE_MANY_COMPOSE` and `DEFINE_MANY_AND` for `shouldEmit`.
Then use helper methods to merge in this strategies