in the process of moving profile-activation, allowed-sources, and version selection into a session object, to try to keep this transient information separate from the underlying data and to provide a single source of control for refining the way the depgraph is used...also to provide a better shot at concurrent access under selection scenarios.
removing session-event support (we may actually need it later, but not now) and adding driver-specific session implementations. Implementing session selection in graph drivers, and fine-tuning the relationships-api especially where sessions and drivers are concerned.

TODO: Implement selection/traversal based on source URI and pom location, which are already captured by the data model and session.
Merge branch 'master' of github.com:jdcasey/atlas