Extract DSL::Evaluator from existing DSLs

Both the Modulefile and Puppetfile DSL classes implemented this
functionality duplicately. This forces standardization of behavior
between all DSLs while DRYing up behavior.
Privatize more DSL::Evaluator methods
Add tomdocs for DSL::Evaluator
Extract DSL::Evaluator#short_class_name
Mark DSL::Evaluator.evaluate and #evaluate as internal methods
Merge pull request #6 from wfarr/dsl-evaluate-refactoring

Extract DSL::Evaluator from existing DSLs