Fixes, fixes, fixes

• Update to ndarray 0.12
• ff-train: show a progress bar while reading the vocabulary.
• Fix a bug in negative sampling for the structured skip-gram model.
• Remove dependency between cloned RNGs.
• Fix one-off in the loss computation, resulting in reporting losses that were too low.
• Add ff-compute-accuracy for evaluating anologies.
• Do not unnecessarily sort the vocabulary upon deserialization.
• ff-train: add the zipf option to specify the exponent of the zipf distribution.