adds scroll_to_row_with_mark to provide 'core' support for new server operation :scrollToRowWithMark


allows testers to scroll to a row with 'mark'

supports searching for cells by:
* accessibilityIdentifier (iOS 5+)
* accessibilityLabel
* subviews in the cell's contentView that match:
- accessibilityIdentifier (iOS 5+)
- accessibilityLabel
- text (UILabel and UITextView)

see https://github.com/calabash/calabash-ios-server/pull/18
see https://github.com/calabash/calabash-ios/issues/128
see https://github.com/calabash/calabash-ios-server/pull/17