LBFGSJob: make maxIterations optional
LogisticRegressionHelper: static implementations of logistic regression
methods

-> TODO this should be merged with LogisticRegression (why do we need to
create an instance of LogisticRegression?)
TrainingErrorJob: basic refactorings
Vectorizer: add weighting scheme as option
CliFrontend: add RCV1 cli for indexing and vectorization