Add basic implicit feedback data support

This adds basic support for implicit feedback data.

- Adds `Plus` event (#517)
- Refactor text-based data access to be more general, deprecating `SimpleFileRatingDAO` and creating new `o.g.l.data.text` package
- Support reading CSV files of unary events, optionally with timestamps
- Support reading CSV files of arbitrary event types with arbitrary fields
Now that we are getting rid of fast iteration, when will we ever need to be able to have plus events that don't extend or override a basic class (and doesn't go through an interface for abstraction?)