Vertical UICollectionViewLayout supporting inline sections (take 2)

[rnystrom]

Working on porting our collection view layout to IGListKit. I'm doing this because its a solid layout, and we just finished preparing it to work with inline sections. It is designed to work in tandem with IGListKit, so we're adding it.

This is still a WIP as I add more tests, but I'd love as much feedback as possible.

Aside from the glob of header documentation, this has the following features:

• Infinite sections that each have infinite items. Sections and items can fall inline. When they break the width of their container they will fall on the next row.
• Sections can have their own insets, line spacing, and interitem spacing.
• Sticky header support! When you use headers, it will always newline the section.
• Maximum width with a border decoration view
  • Use this to pinch in your content on larger devices

Followup to #423

TODO

• Move decoration view support to delegate removed
• Unit test changing top y sticky inset
• Update initWithCoder: to use sensible defaults
• Test initWithCoder: throws
• Test when remaining cells outside of visible bounds
• Move decorationBorderViewOfKind: internal? C function? (or add unit tests) removed
• Unit test changing bounds size

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Copied from #450

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@rnystrom updated the pull request - view changes

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