Transaction on new objects

Issue #10 resolved
Jean-Tiare Le Bigot
created an issue

At the moment transactions may only involve existing objects as the engine relies on the expected_value feature of save.

In v1.7.0, thie parameter will be replaced by raise_on_conflict thus allowing smater behavior to take place in DynamoDBModel. save will then auto-detect that the object is not comming from the database and internally replace the expected_value by a allow_overwrite=False