Fixed a crash when switching between layouts and/or different data sources.
Merge pull request #153 from pinterest/Layout_Update_Crash_Fix

Fixed a crash when switching between layouts and/or different data sources.
Fixes deleting sections.

Signed-off-by: Daniel Thorpe <daniel.thorpe@corp.badoo.com>
Merge pull request #156 from danthorpe/delete_sections

Fixes deleting sections.
Applied LayoutAttributes to Cell/SupplementaryView at dequeue time as opposed to configuration time.
Optimised images
Merge pull request #155 from pinterest/Layout_Attribute_Application_Fix

Cell LayoutAttributes Should be Applied Before "cellForItemAtIndexPath:"
Merge pull request #159 from imageoptimiser/optimised-images

Optimise images (1 MB reduction)
fixes a typo
Honor the z-index of PSTCollectionReusableView's when adding them to the PSTCollectionView
Merge pull request #160 from pinterest/aly_view_ordering

Honor the z-index of PSTCollectionReusableView's when adding as subviews to PSTCollectionView