[2.0] DDC-336 - Support specification of an OrderBy SQL snippet in OneToMany and ManyToMany Associations in Annotation, XML and YAML Drivers

[2.0][DDC-284] Fixed. API polish and some convention over configuration simplifications for join columns and join tables.

[2.0] DDC-271 - Add columnDefinition for Join-Column in Annotation, XML and YAML driver, updated SchemaTool to use Join Column definition instead of using the column's columnDefinition if specified. Refactored MappingDriverTest to allow for feature specific test-cases instead of one large assertion block. Fixed typos in XmlDriver - Updated doctrine-mapping.xsd Schema File

[2.0] DDC-278 - Add missing "columnDefinition" attribute to field tag of XML Schema for Doctrine Mapping.

[2.0] Fixed several referential integrity issues. Fixed critical issue with inserts being run twice on postgresql/oracle. Added support for additional tree walkers that modify the AST prior to SQL construction and started to play with it in a testcase.

[2.0] Refined implementation and semantics of the merge and detach operations. General cleanups and API improvements. Added a testcase for detaching/serializing->unserializing->modifying->merging to demonstrate the transparent serialization.