Add support for -tap on UICollectionViewCell

Analogous to -tap on UITableViewCell.

Note: Test for "-tap for a multiple selection collection view should not deselect the cell if another cell is tapped" asserts that `[controller.collectionView indexPathsForSelectedItems]` contains the relevant index paths rather than asserting equality of the returned array with `@[relevant index paths]`; this is because when running the specs via rake, `-indexPathsForSelectedItems` returns the array in reverse order for some reason, causing failure if the test asserts equality on the arrays.