Better serialization for Tokenizer and Config classes (BERT, GPT, GPT-2 and Transformer-XL)
This PR add standardized serialization to all the tokenizers (BERT, GPT, GPT-2, Transformer-XL) through a `tokenizer.save_vocabulary(path)` method.

Also add a serialization method to all the Configuration classes: `Config.to_json_file(file_path)`

Added clean examples for serialization best practices in README and examples.

Also fixes Transformer-XL "split on punctation" bug mentioned in #466.