Parallelization

[mbillingr]

Random forests are particularly well suitet for parallel processing. Trees are independent and could be built in separate threads. The same is true for prediction, but this is usually faster than building, so there may be less benefit.

The current implementation mutates the training data. This is not nice for threads, but copying the data once per thread may not be such a big deal. Bootstrapping (#25) will come with additional copies/references anyway.