if there is already a cell present the estimated height will be that actual height
using a NSMapTable instead of an Array
added load more table view cell height constant
box notation instead of numberWithFloat
Modified height-estimation to not rely on story identifiers.

Changes suggested by @bskinner
Need to make sure index path is a NSIndexPath.

Sometimes UIMutableIndexPath are returned and sometimes NSIndexPath are returned.  These two items will never equal each other.
Abstracting getting and setting of cached height rather than abstraction of creating the key.
Relabeled methods

Removed class checking on NSIndexPath

Suggestions from @bskinner
Remove cache if stories are refreshed.
Merge branch 'refs/heads/3.6/ipad-news' into 3.6/ipad-news-tableview-height-estimation
Using new NSIndexPath category.

Changes suggested by @bskinner
Tweak the removal of cached heights
Whitespace
Switch over to a semaphore instead of a lock
Remove the objectIdentifiers set, it is not used
Clean up the objects accessor
Fix the associated object key
Rename the FRC update method and add error checking to the categories accessor
Setup the superclass to track and manage the contexts
moved cachedHeights to superclass