Added a test to verify the ability to create a dict with keys that are elements.
Added a test to verify the ability to create and read non-persisted attributes.
Added the delete part of the flush operation.
Release all caches when refreshing.
Make sure there is no duplicates in the session identity maps.

We need to use this dirty iteration because the "in" operation triggers
a call to __eq__ that is overloaded in bulbs.element.Element, which
causes an error.
Removed extra statement.
Merge pull request #7 from adurieux/staging

Implemented deletion in the flush() operation.