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
The [getTimestamp()](http://lenskit.org/apidocs/org/grouplens/lenskit/data/event/Event.html#getTimestamp--) JavaDoc.