[2.0] DDC-239 - Added several new mapping validations for duplicate association/field and or column name definitions, and in combination with the discriminator column name.

[2.0][DDC-92] Fixed. Patch provided by Christian Heinrich. [DDC-274] Started some method renaming. [DDC-142] Fixed (join column names and discriminator column names dont support quoting) [DDC-258] Fixed.

[2.0][DDC-265] Partial support to inheritanceType in AnnotationExporter. Still needs to add the discriminator column.

[2.0] DDC-268 - Exchanged DoctrineException for MappingException and added missing exception method (thanks to Christian Heinrich for the patch)

[2.0][DDC-254] Improved MappingException thrown on missing required configuration of Many-Many mapping

added missing annotation driver class. small refactorings. added first ClassMetadata serialization tests.