Add ability for weighted code

[mayhewsw]

Add ability for learners to be weighted. See this branch of my fork: https://github.com/mayhewsw/lbjava/tree/weighted

The difficulty is that the abstract function learn() in Learner.java needs to be changed, which is a breaking change. It's easy to retrofit classifiers, but it is busy work (already done in my branch).

So far, I have implemented weighted learning only for SparseAveragePerceptron.

Any comments @mssammon @cowchipkid @danyaljj ?