Add full language model fine-tuning
These scripts add language model fine-tuning that closely mirrors the training process in the original BERT repo. The old fine-tuning example has been renamed `simple_lm_finetuning.py`. The key difference is the old script did not merge sentences when creating training examples, and so tended to create short training examples with lots of padding tokens - this is explained more fully in the README. The new scripts follow the BERT repo approach, which concatenates sentences from both documents to pack the training example up to `max_seq_len`.

All the scripts for LM fine-tuning have been moved to a subfolder of `examples/` with an included README to explain what LM fine-tuning is and how to use them.
Also, build_py2 has failed because I deliberately did not include Py2 compatibility - it's only a few months from end-of-life now. If you really want me to, I can go back and include it, but we should be trying to let it go by now!<|||||>This is really great @Rocketknight1!
Thanks for taking the time to make a very clear and informative README.
I've fixed the formatting issues in the README, slightly re-worded it and added a link to it in the main README.
I'm merging it now.
Great job!