Codex version 0.3.0.

Release Notes

• Add initial MySQL visitor.
• Implement UNION, INERSECT, and EXCEPT for Select statments.
• Rename managers SetEngine method to Engine.
• Make managers engine member private to enforce using Engine method to modify it.
• Remove near useless type switch in favor of more recognizable type if.
• Implement Having and Group By for Select Managers.
• Consistent naming convention for receivers.
• Consistent returns from factory methods.
• Select Statement ordering with Ascending and Descending nodes.