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.