Fix documentation.
Fix history import_dataset() when run after a tool.

An additional history.refresh() before

  old_ids = set(history.dataset_ids)

would be needed, but the code can be simplified now.
Remove from ObjHistoryClient the update() method which should be called from History instances (objects cleanup).
Remove from ObjHistoryClient and ObjLibraryClient the get_dataset() and get_datasets() methods (objects cleanup).
Remove from ObjHistoryClient the import_dataset() method which should be called from History instances (objects cleanup).
Remove from ObjDatasetClient the get_stream() method which should be called from Dataset instances (objects cleanup).
Better formulation for testing dataset container suggested by Simone Leo.
Merge pull request #81 from nsoranzo/objects_cleanup

ObjHistoryClient and ObjDatasetClient cleanup