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.