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
Would this also be clearer if we had `Plus` (or `Like`) and `PlusBatch` as distinct kinds of things, and an 'amplified Plus' would need to be something else entirely?