Summary:
Adding a pretty basic stacked demo. 3 different sections, each with 3 child section controllers that handle an `Int` data.
Requires #354 to be fully functional (working range).
Closes#134
- [x] All tests pass. Demo project builds and runs.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Closes https://github.com/Instagram/IGListKit/pull/355
Differential Revision: D4363859
Pulled By: jessesquires
fbshipit-source-id: 5dbc8e18388fed4930f189e3477ff57f0edf2d13
