Fixes crash when running a unit test bundle while an app bundle runs.

objc_allocateClassPair returns nil if the class pair already exists.
Calling objc_registerClassPair with a nil argument causes an EXC_BAD_ACCESS.
Merge pull request #311 from sinnerschrader-mobile/master

Fixes a crash when setting up a unit test environment
Fix comparison of different signs
Fix implicit float/double conversion
Use old collectionViewData when iterating through a deleted section
Merge pull request #313 from JensDee/features/fixWarnings

fix warnings
Merge branch 'master' of https://github.com/steipete/PSTCollectionView
Merge pull request #314 from alexanderedge/master

Items not removed from view when deleting sections