fixed problems with joined inheritance and composite keys

SchemaTool now creates all Id columns not just only the first one.
Insert statement for child entity now contains parameter for additional key columns only once.
inheritance with composite keys

added tests for single table inheritance
Adding EntityManagerDecorator base class as an extension point for EntityManager
Fixed entities path

In the tutorial, the user is told to create a new file in the '/src' folder, and the 'entities' folder is never referenced. Updating the SQLite schema according to the tutorial fails with the 'No Metadata Classes to process.' message. Changing the folder to '/src' fixes this, ostensibly.
Merge pull request #652 from dannykopping/patch-1

Fixed entities path
Fix DDC-2415
Merge pull request #653 from FabioBatSilva/DDC-2415

[DDC-2415] Fix CustomIdGenerator inheritance
Fix DDC-2409
[DBAL-483] Pass default values to DBAL mapping layer correctly to fix default comparision bug.
Merge branch 'DBAL-483'
Merge pull request #655 from FabioBatSilva/DDC-2409

[DDC-2409] Fix merge association STATE_NEW
Simplify condition of previous commit (5cdc73e)
[DDC-2106] Fix entity as parameter value when its managed but not yet with identifier.
Merge branch 'DDC-2106'
[DDC-1984] Throw exception if passing null into UnitOfWork#lock() - which can happen when EntityManager#find() tries to lock entity that was just deleted by another process.
Merge branch 'DDC-1984'
[DDC-1998] Pass types to Connection#delete() to allow custom conversions to happen.
Merge branch 'DDC-1998'
[DDC-2136] Fix exporting to YAML and XML with assocation keys.
Fix Docs build