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 don't think this is clear enough. To me this reads as "-t <filename>" where <filename> is the name of a file containing type data. I don't think that is what you mean.

how about something more like "the type of events in the input file. default <rating>"?