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
This is just a move of a class used to implement the old `SimpleFileRatingDAO` and, as such, is not expected to be used in new code. Perhaps we shouldn't even move it, and/or should deprecate it.