switched to using fast iterators where possible on two metrics.
Merge https://github.com/grouplens/lenskit
Merge https://github.com/grouplens/lenskit
added a call to CollectionUtils.fast() to the entropyMetric and popularity metric
Merge https://github.com/grouplens/lenskit into kluver/fast-iterators-in-eval
cleaned up prefixes and suffixes in all topN metrics, all metrics now support prefixes and suffixes, and the generic topN metric builder now supports prefixes and suffixes (avoiding a lot of replicated code). Additionally, removed some supuerfluous and error causing code from TopNEntropyMetric
fixed suffix generation. added tests for prefix and suffix generation.
fix licenses
documented prefix and suffix behavior in the builder object.
Merge pull request #575 from kluver/kluver/clean-up-topN-evaluators

Clean up Top-N evaluators and fix NPEs.