Add parallel dataprocessing

[Timoeller]

This PR addresses #26
Multiprocessing works well and fast.
Problems:

• When logging 3 random Samples it prints sample.features = None, but in debugging it has features and also starts training the NN....
• Tqdm progressbar is missing in parallel mode
• It OOMs when calling _featurize_samples() in processor.py on full SQuAD dataset.
  I remember having similar issues (using a lot of memory but not OOM) with a single process on SQuAD, too.
  Please review