Report mean and stdev for accuracies of trained classifiers

[dbogdanov]

Provide stdev for accuracies of trained classifiers in addition to mean across folds when reporting n-fold cross-validation results. Currently, accuracy is computed from the overall confusion matrix which is a sum of confusion matrices for each fold.

The evaluateNfold should gather all confusion matrices apart in a list and return that list.

[alastair]

Is this the same as #21 ?

[dbogdanov]

Implemented in 49517aa (PR #86)

[dbogdanov]

We won't store confusion matrices for each train/test split in cross-fold validation, as they clutter results output. The split can now be reproduced as we store the seed used for the random split.