Rule versioning artifact

[sharwell]

This change implements #35 as an optional separate artifact for users that wish to incorporate its static analysis features.

• Implement rule versioning support in a separate Maven artifact
  • Add the RuleDependency annotation interface for specifying a rule dependency in code, and the RuleDependencies annotation interface to allow application of multiple @RuleDependency annotations to a single element in code.
  • Add the Dependents enumeration for specifying complex relational dependencies
  • Implement an annotation processor for validating rule dependencies
• Restrict changes in the Tool and reference runtime itself to the minimal code necessary for implementing this feature:
  • Add the RuleVersion annotation interface
  • Add the -rule-versioning command line argument to the tool (disabled by default)