Let 'waitForCellAtIndexPath' actually wait for a cell at a given index p...

...ath

This pull request gives more flexibility when dealing with tableviews and collectionviews.
In our application we usually show an empty tableview, where the content is updated through a web request.
Currently on KIF the method waits only the tableview, assuming that if we have the table view, we should have also the cell, that's not always true, indeed in our application it always result on "cell not found" error.
I expanded the method to actually wait for an existent cell at a given index path.