Fix and test averaged perceptron
Add failing test for overflow and fix init bug
Use online average computation
Allow delayed averaging.

I remember reading somewhere that sometimes it's good to wait ~5 epochs
before starting averaging.  @mblondel recommends averaging over the final
epoch, but I haven't gotten around to implementing that one yet.
Add perceptron reference (Collins, 2002)
Fix tiebreaking in binary SVM and test for it
Implement averaging over final epoch
Test for correct behaviour when averaging and early stopping happens