[DBAL-722] Introduce subclasses for runtime relevant exceptions (NotNull, DuplicateKey, ForeignKey Constraint Violation).

- Fixed docblocks
- Removed return statements on void methods
- Cleaned up unused private / local variables
- Fixed license issues
- Removed SVN leftovers
- Fixed doctrine-project.org links
- Added line breaks before return statements

[2.0][DDC-237][DDC-216] Fixed. If you're using manual proxy generation through the CLI, please regenerate your proxies.

[2.0][DDC-164][DDC-165] Fixed. Cleaned up ManyToManyMapping. Cleaned up identifier handling and handling of composite identifiers in some places.

[2.0][DDC-49][DDC-50][DDC-33] Fixed DDC-49 and DDC-50. Also addressed DDC-33. Various other small changes and fixes. For problems with proxy objects please refer to the updated documentation.