Summary:
I started working on adding support for dequeuing cells created from nibs (issue #1). Additionally I extended `IGListSingleSectionController` so that it can be used with nibs too. I don't know if you had this also in mind.
- [x] I'm currently thinking about the best way to test these changes.
- [x] I was not able to update the documentation (issue #55).
- [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/CONTRIBUTING.md)
Closes https://github.com/Instagram/IGListKit/pull/56
Reviewed By: dshahidehpour
Differential Revision: D4023746
Pulled By: rnystrom
fbshipit-source-id: 6a8b4cfb4dba38ea6e9870a9a4506288ee155cfe
