Convert recommender engine tests to Groovy
Rewrite engine validation logic (#459)

- Allow engine validation to be deferred
- Mark engines as instantiable or not
Support late rewriting (#459)

Add `createRecommender(LenskitConfiguration)` method.
Add missing license header
Add parameters to control the file DAO
Merge pull request #461 from elehack/feature/elehack-459-many-rewrites

Support late rewriting (closes #459)