Make -tableView:heightForRowAtIndexPath: faster and linear in time, refs #6

Mostly an implementation of DwightEverhart sample code (https://github.com/square/objc-TimesSquare/issues/6#issuecomment-13690845)
Fix typo and update the documentation, refs #6
Merge pull request #14 from kemenaran/optimize-height-calculations

Make -tableView:heightForRowAtIndexPath: faster and linear in time