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
Assuming I know what service loader does (thats what manifest files do right?) I think this is really cool.
Will anyone thats not you (or maybe me) 
1) know that they can use this manifest magic?
2) know how to use this manifest magic?
3) want to use this magic over explicitly configuring something?