add toString methods to similarity functions
improve logging of item-item build process
Build item-item build context with a provider
Implement sparse item-item neighbor iterator
Refactor neighbor iteration into a strategy

This makes neighbor selection for the default item-item model
builder a strategy with sparse and non-sparse implementations.
Update the release notes for #432.
Merge pull request #432 from grouplens/feature/elehack-iibuild-cleanup

Logging and performance improvements to the item-item builder