Create private method in IGListStackedSectionController to get relative index
Create private method in IGListStackedSectionController to get relative index

Summary:
Fixes from PR #273 rebase.

jessesquires rnystrom this should look a little better and be able to merge.

- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Closes https://github.com/Instagram/IGListKit/pull/275

Differential Revision: D4271440

Pulled By: rnystrom

fbshipit-source-id: a85b8c80b5504640d7f82b183ed5b5d0d0ea5c1b

dshahidehpour/IGListKit,dshahidehpour/IGListKit,dshahidehpour/IGListKit,zhubofei/IGListKit,charles0126dev/IGListKit,charles0126dev/IGListKit,Instagram/IGListKit,Sherlouk/IGListKit,zhubofei/IGListKit,Sherlouk/IGListKit,Instagram/IGListKit,zhubofei/IGListKit,Sherlouk/IGListKit,charles0126dev/IGListKit,zhubofei/IGListKit,Instagram/IGListKit,Instagram/IGListKit,Instagram/IGListKit,zhubofei/IGListKit