fix bug #521 in TopNMetricBuilder class
untested code for computing the precision and recall of a recommender output.
untested code for computing the independent recall of a recommender
untested code for computing the independent recall of a recommender  (rest of code from last commit)
added a few more itemSelectors in support of using the TopNIndependentRecallMetric to replicate previous work
updated capitolization on string constant
Merge https://github.com/grouplens/lenskit into feature/kluver-415-percisionRecallMetrics
changes for code review. Largely renameing two classes and refactoring out shared code.
Merge pull request #545 from kluver/feature/kluver-415-percisionRecallMetrics

Merge the top-N precision/recall code (closes #415, closes #521)