Added two pre-trained models and one new fine-tuning class

This release comprise the following improvements and updates:

• added two new pre-trained models from Google: bert-large-cased and bert-base-multilingual-cased,
• added a model that can be fine-tuned for token-level classification: BertForTokenClassification,
• added tests for every model class, with and without labels,
• fixed tokenizer loading function BertTokenizer.from_pretrained() when loading from a directory containing a pretrained model,
• fixed typos in model docstrings and completed the docstrings,
• improved examples (added do_lower_caseargument).