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
I suppose. It just looks werid when I see `RatingBuilder implements EventBuilder<Rating>, Builder<Rating>`. But if we used to use the Builder interface, but don't intend to now, then lets leave it as is.