Small improvements and a few bug fixes.

Improvement:

• Added a cache_dir option to from_pretrained() function to select a specific path to download and cache the pre-trained model weights. Useful for distributed training (see readme) (fix issue #44).

Bug fixes in model training and tokenizer loading:

• Fixed error in CrossEntropyLoss reshaping (issue #55).
• Fixed unicode error in vocabulary loading (issue #52).

Bug fixes in examples:

• Fix weight decay in examples (previously bias and layer norm weights were also decayed due to an erroneous check in training loop).
• Fix fp16 grad norm is None error in examples (issue #43).

Updated readme and docstrings