Adds support for multi-class linear SVMs.

This patch
  * Adds a one-vs-all meta classifier into meta/
  * Adds a LinearSVC (essentially the same as LogisticRegression
    but with different libsvm parameters) to linear_models/
  * Adds a MultiLinearSVC into ensemble/ for predicting
    CategoricalAttribute  classes with the LinearSVC
  * Adds a new example dataset based on classifying article headlines.

The example dataset is drawn from WikiNews, and consists of an average,
min and max Word2Vec representation of article headlines from three
categories. The Word2Vec model was computed offline using gensim.
Merge pull request #88 from Sentimentron/linearsvc-staging

Multi-class linear Support Vector Classifiers