Fixing docstring on Dataset.get_entities.
Merge pull request #450 from dhermes/fix-docstring-error

Fixing docstring on Dataset.get_entities.
Fix regression test failure introduced in PR #449.
Merge pull request #452 from tseaver/449-fix_transaction_borked_in_449

Fix regression test failure introduced in PR #449.
Re-writing Entity to subclass object instead of dict.