Write test for root-anchored matchers (#344)

This test verifies that root-anchored context matchers work.
Implement recommender engine builder

- Create LensKit recommender engine builder, providing API points for #449 and #337
- Factor out recommender engine building to new objects
Factor out recommender engine loader (#337)
Implement graph rewriting (#337)

- Add rewriting to engine builder
- Add rewriting to engine loader
- Re-enable rewriting-based tests
Update FunkSVD tests for rewriting (#337)

- Convert to Groovy
- Re-enable skipped tests
Update least-squares tests for rewriting (#337)
Update user-user tests for rewriting (#337)
Pull DAO config up to base test suite
Update item-item tests to Groovy & rewriting (#337)
Add instance-based addComponent (#457)
Add class-based addComponent (#457)