Clean up the error domain and code definitions
Return an error if there is already a request in progress
Merge pull request #140 from MIT-Mobile/3.6/news-data-source-callbacks

3.6/news data source callbacks
Merge remote-tracking branch '3.6/ipad-news' into '3.6/data-source-notifications'

Conflicts:
	Modules/News/Model/MITNewsCategoryDataSource.m
	Modules/News/Model/MITNewsStoriesDataSource.m