[2.0][DDC-30] Fixed null values with Date, Time and DateTime types. Thanks Ismo for report and patch

[2.0] Various necessary fixes and adjustments for oracle as well as fixes for better handling of temporal types. Closed #2452.

[2.0] More docblocks. Renamed methods Type::addCustomType to Type::addType and Connection::exec to Connection::executeUpdate. Added Type::hasType.

[2.0] Streamlined result structures. Hydration improvements: Small overall improvements, large improvement for (joined) fetched collections (~60%). 

[2.0] Implemented DQL bulk DELETE support for Class Table Inheritance. Other cleanups, refactorings and docblock additions.

[2.0] Introduced SQL logging facilities. Made Type constructor private to prevent instantiation and force use of the factory method getType().

[2.0] Fixed issues discovered during code coverage report generation. Made all ClassMetadata properties public (good reasons, read the class docblock). Performance improvements.

[2.0] Started to implement proxy object handling. Introduced a configuration switch for partial objects (allowPartialObjects) that defaults to TRUE. Setting to FALSE enables proxying and lazy-loading.

[2.0] Moved code between Query and AbstractQuery. Added first NativeQuery implementation. Hydration work and code movements for discriminator column usage. Started implementing Single Table Inheritance.