better error message
create superdirectories for files to be put
check whether training returned a model
Merge branch 'devel' of https://github.com/NPSVisionLab/CVAC into devel
K's changes for balanced svm

Below is additional information about the problem of im(un)balanced data

Paper
    - http://www.ele.uri.edu/faculty/he/PDFfiles/ImbalancedLearning.pdf

Various issues and solutions
    - http://www.reddit.com/r/MachineLearning/comments/12evgi/classification_when_80_of_my_training_set_is_of/
    - http://stackoverflow.com/questions/2284059/svm-classification-minimum-number-of-input-sets-for-each-class
    - http://stackoverflow.com/questions/11736125/how-do-you-handle-data-imbalance-in-svm
    - http://stats.stackexchange.com/questions/46022/libsvm-cost-weights-for-unbalanced-data-doesnt-work
    - http://stats.stackexchange.com/questions/20948/best-way-to-handle-unbalanced-multiclass-dataset-with-svm

(temporary) Solution
    - Weighted SVM: see the line 419 in bowCV.cpp