Instead of using 1 dictionary for all index paths we use N dictionaries with N being the number of sections.

We were looking if the numberOfStoryHeights > numberOfStoriesForCategoryInSection but all categories do not have the same number of stories.

Fixed #165 #166.
Removed cached heights that are not in tableview after reload.
initialize dictionary with nil
Merge pull request #168 from MIT-Mobile/3.6/ipad-news-estimatedHeightForRowAtIndexPath

3.6/ipad news estimated height for row at index path. Fixes #166