Set indexPath regardless of if it is present in the new model
Assemble a dictionary of views that are to be removed

The previous implementation would check if a given PSTCollectionViewItemKey is
present in the new visible views dictionary. This did not work for the following
case:

Item [0,0]
Item [0,1]
Item [0,2]

Deleting item at [0,1] should result in the alpha fade and eventual removal
from the superview and recycle, but actually since [0,1] is a valid index in the
new layout, [0,2] gets removed instead.

This solution adds views that are to be deleted into a specific NSDictionary
that gets enumerated after all animations are finished. All views contained
inside are recycled.
Add array backing to illustrate deletion
Fix compiler warning - unused variable
Fix double 'batchUpdates'
Uncomment removing deleted view from visible views dictionary

Since `previouslyVisibleViewsDict` is no longer used, we can safely remove
the deleted view from `_allVisibleViewsDict`.
Merge pull request #275 from alexanderedge/master

Delete item causes display corruption in flow layout