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
Maybe, if we want to keep using `Builder`.

Right now, we don't use `Builder` consistently, or really take advantage of using it at all. I would be inclined to stop implementing it until we actually have a use case for using it as an interface.