UITableViewの最後のセルのIndexPathを見つける ref: http://qiita.com/noppefoxwolf/items/05c4028d0c3fb6b44fef

file0.swift

extension UITableView {
  var lastIndexPath: IndexPath? {
    guard let dataSource = dataSource else { return nil }
    return dataSource.lastIndexPath(in: self)
  }
}

extension UITableViewDataSource {
  func lastIndexPath(in tableView: UITableView) -> IndexPath? {
    guard let sectionCount = self.numberOfSections?(in: tableView) else { return nil }
    let lastSection = sectionCount - 1
    let rowCountOfLastSection = self.tableView(tableView, numberOfRowsInSection: lastSection)
    let lastRow = rowCountOfLastSection - 1
    return IndexPath(row: lastRow, section: lastSection)
  }
}