Add sub-navigation by stack

This builds on #18, so it might be worth viewing https://github.com/guardian/status-app/compare/pw-case-classes-for-serialization...stacks?expand=1 if you just want to see the Stack specific work.

@jennysivapalan does this look sensible:

![image](https://cloud.githubusercontent.com/assets/68329/2983180/59d48138-dc1f-11e3-966b-105f93a666ab.png)