Fixed Averaged Perceptron.

There was a mistake in the update for the averaged perceptron. I also
made a change so that the averaging was based on the weighted updates,
not just the number of updates.
Fixed subset_keywords and training features

Subset keywords did not explain things properly in the help. Create
training features did not handle subset keywords properly. If I had a
keyword definition file that did not contain a keyword in the training
data, it was not given a target of 0.
Added Weighted Binary Linear Model

Began the process of building a single binary linear model class.
Currently the perceptron and averaged perceptron have been converted to
this framework. Now it should be straightforward to incorporate other
types of models (Naive Bayes, Logistic Regression).
Finished Logistic Regression.

Fixed the averaged perceptron. Added the linear regression, with and
with regularization. Removed the separate perceptron class. Now
everything is in a single class.