Fix aggregation with no predictions (#401)

- Add userRow() and finalRow() convenience methods to help
  build user/final result rows more easily
- Return null/NA when no users are scored in a metric
Make Pearson correlation use mean of common items

Pearson correlation used to offset from the mean of all items, not just common items (#404). This fixes that.
bump external build to 0.1
[maven-release-plugin] prepare release lenskit-2.0.3.2
[maven-release-plugin] prepare for next development iteration
add 2.0.3.2 release notes
Backport #358 fix from master (46eb5ff)
don't ignore data dirs
Refactor TT data builder

- Add newBuilder methods to data set class
- Add copyBuilder method
- Simplify preference domain semantics for train-test builders
- Update simple evaluator for this
Add query set to TT data sets
Make DAOs extend common interface
Bind query DAOs in configuration

- Add QueryData annotation for query (not train) DAOs
- Bind QueryData DAOs in eval config
Add explicit toInstance binding ability
Mark mean-centering normalizer as shareable
Drop active task note
Log successful task completion
Add 2.0.4 release notes
Back out ee5f1c7 (common DAO interface)
[maven-release-plugin] prepare release lenskit-2.0.4
[maven-release-plugin] prepare for next development iteration