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.
Merge pull request #315 from mattjgalloway/speed-up-large-datasets

Improve speed, especially with large data sets