Clean up some test comments.

Galaxy tests had copied and pasted CloudMan test comments.
Add test for show_dataset in histories.

Some refactoring to support this.
Add delete_dataset method on HistoriesClient.

With test case.
Add method for updating history dataset.

With simple test case.
Create/Read/Update/Delete operations for history dataset collections.

API operations, test cases, and small set of utility classes to ease building up collections.