Emphasize defining selectors alongside reducers in examples

[gaearon]

We should use selectors in our examples like we do here: voronianski/flux-comparison#64.

Otherwise people are having a hard time how to change the state shape safely because they have to update their mapStateToProps all over the project.

[barockok]

@gaearon should we start use reselect ?

[gaearon]

Not necessarily. I mostly mean the idea of exporting selector functions from reducer. They don't have to be powered by reselect (although it's a nice optimization).

In fact we already moved shopping cart example from Flux comparison into Redux repo, so this issue can be closed, as it adequately demonstrates the technique.