Add UITapGestureRecognizer to contentView instead of cell itself

This allows the add and delete buttons that are added by
`UITableViewCellEditingStyleInsert` and
`UITableViewCellEditingStyleDelete` to intercept touches.
Merge pull request #82 from bartvandendriessche/fix/add_tapgesturerecognizer_to_contentview

Add UITapGestureRecognizer to contentView instead of cell itself