Initial stab at implementing the ability to select a session to use per
model.

Changed session_name accessor, it was conflicting.

Attempting to fix a stack-frame bug.

Further incremental improvements to method definitions.

Fixing method call, it's a class method not an instance method.
Fixed unit tests that were failing due to the new neo4j_session method.
Merge pull request #299 from undergroundwebdesigns/multi-session-model-support

Initial stab at implementing the ability to select a session to use per model.