llama_0.7.1:
- allow vbs/singleBest to operate on test splits to simplify the interface
- corrected the implementation of contributions() to handle minimisation and
  maximisation of performance values correctly


llama_0.7:
- add regressionPairs model, which predicts the performance difference for each
  pair of algorithms and makes decisions based on that
- use mlr for machine learning algorithms
- use original problem features along with predictions in stacked learners
