Various 2.1 fixups

Some small improvements and fixups for LensKit 2.1.

- Allow cheating (#600)
- Abstract rating writing for crossfolding and subsampling, allow crossfold/subsample directly to binary packs (#587)
- Make `get` methods work better (#596)
- Bump annotation processor source version (#569)
- Redo SimilarityMatrixModel constructor (#523)
- Add MathJax footer to JavaDoc (#592)