Changed signature of Model and ModelCommand.
Addressed User.
Addressed Article.
Addressed Keyword.
Addressed Bookmark.
Addressed CompositeModel.
Removed CompositeEnumerableCommand.
Addressed CompositeModelCommand.
Renamed CompositeModelCommand to CompositeCommand.
Addressed ModelCommand.
Removed NullCommand.
Addressed ConditionalCommand.
Addressed DeleteBookmarksCommand.
Addressed DeleteCommand.
Addressed DeleteConfirmableCommand.
Addressed DeleteKeywordsCommand.
Addressed InsertCommand.
Addressed InsertConfirmableCommand.
Addressed RelayKeywordsCommand.
Addressed SelectBookmarkedArticlesCommand.