add new token classification model

pytorch_pretrained_bert/__init__.py

 from .tokenization import BertTokenizer, BasicTokenizer, WordpieceTokenizer
 from .modeling import (BertConfig, BertModel, BertForPreTraining,
                        BertForMaskedLM, BertForNextSentencePrediction,
-                       BertForSequenceClassification, BertForQuestionAnswering)
+                       BertForSequenceClassification, BertForTokenClassification,
+                       BertForQuestionAnswering)
 from .optimization import BertAdam
 from .file_utils import PYTORCH_PRETRAINED_BERT_CACHE