Summary:
Adding a working range example to the example app. This example:
- Displays a list of images downloaded from unsplash.it
- Create 20 uniquely-random sized objects
- When section controllers enter the range, create a data task to download the image
- When finished, store image in `downloadedImage`
- Set in cell if cell is visible
- Don't create task if already downloaded or task created
- Cancel task when section controller is destroyed
- Show a spinner cell while downloading the image
Fixes#84
- [x] All tests pass. Demo project builds and runs.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/CONTRIBUTING.md)
Closes https://github.com/Instagram/IGListKit/pull/89
Differential Revision: D4050210
Pulled By: rnystrom
fbshipit-source-id: 628a777fa819dccd9d9f4f58646cdf72fb4bc65c
