Add assertion that query cursor is copied during '_clone'.
Rework Query._clone to avoid spurious copy of dataset.

Fixes #171.
Update docstring for 'Connection.save_entity'.

Indicate that upsert clears existing properties before applying those
passed to the API.

Fixes #167.
Unused import.

Incorporate feedback from @dhermes.
Update doctstrings for 'Entity.save' and 'Connection.save_entity'.

Reinforce that previously-stored properties which don't have values set
in the current entity instance will be removed.
Clarify docstring for 'Entity._must_key'.

Incorporates feedback from @dhermes.
Merge pull request #258 from tseaver/167-connection_save_entity

Fix #167:  Update docstring for 'Connection.save_entity'.
Merge pull request #253 from tseaver/174-verify_entity_repr

Fix #174: verify Entity.__repr__ logic
Merge pull request #254 from tseaver/171-rework_query_clone_deepcopy

Fix #171: rework query clone to avoid spurious copy of dataset
Merge branch 'master' into 168_169-connection_delete_entities