Use reuseIdentifier in Decoration View ItemKeys
Improve speed, especially with large data sets

_globalItems is not a cache of the index paths (at least, in UICollectionView it isn't). This commit
removes that concept for now. Eventually, it should be added back in, but when it does there are a
whole load of other methods that need adding such as 'loadEverything' and
'setLayoutAttributes:atGlobalItemIndex:'. We should do that, but that's for another day.
Lazy load the collection view for registering cells at init
Merge pull request #316 from mattjgalloway/lazy-load

Lazy load the collection view for registering cells at init
Merge pull request #315 from mattjgalloway/speed-up-large-datasets

Improve speed, especially with large data sets
Merge pull request #256 from cysp/for-review/decorationview-itemkey

DecorationView "kind" identifiers are ignored
Add missing import
Rename const string
Add Decoration View test case (thanks to @cysp)