Refactored out blueprints dependency from core module.

- Created dedicated blueprints module for blueprints graph base classes
- Reimplemented HeapGraphDatabase without dependency on blueprints
- Added HeapGraphDatabaseStressTest
Merge branch 'blueprints' of github.com:jdeolive/GeoGit into jdeolive-blueprints
Refactored out blueprints dependency from core module.

- Created dedicated blueprints module for blueprints graph base classes
- Reimplemented HeapGraphDatabase without dependency on blueprints
- Added HeapGraphDatabaseStressTest
Merge branch 'jdeolive-blueprints'