1、更新requirements以及README.md
2、更新DataLoader
3、更新loss
4、更新model/bert.py内容及适配的测试代码
5、更新reproduction/README.md
6、修复其他测试代码的报错的地方

2. 修改embedding.py中的bug
3. ConllReader默认跳过所有的DOCSTART标签
4. 交换bert的heavy lifting到_bert, 将BertEncoder在bert.py中暴露
5. crf中allow_transition的include_end_start修改为false，以与CRF的默认值适配
6. allow_transition与SpanMetric支持BIOES类型的tag
7. datainfo中增加打印格式化输出

- fix tests

# Conflicts:
#	fastNLP/core/callback.py
#	fastNLP/core/trainer.py

2. metric增加SpanFMetric，可以用于计算sequence labelling的performance
3. 分词复现任务根据新版接口做了部分调整。

* refine code style
* set up unit tests for Batch, DataSet, FieldArray
* remove a lot of out-of-date unit tests, to get testing passed

- DataSet's __init__ takes a function as argument, rather than class object
- Preprocessor is about to remove. Don't use anymore.
- Remove cross_validate in trainer, because it is rarely used and wired
- Loader.load is expected to be a static method
- Delete sth. in other_modules.py
- Add more tests
- Delete extra sample data

2. changes of names:
  aggregation ----> aggregator
  interaction ----> interactor
  action.py ----> sampler.py
  BasePreprocess ---> Preprocessor
  BaseTester ----> Tester
  BaseTrainer ----> Trainer
3. add more code comments
4. fix bugs in predictor's data_forward
5. in sampler.py, remove Bachifier, fix some codes. but not test
6. remove unused codes in other_modules.py & utils.py
7. update fastnlp.py with new config file names and code comments
8. add data examples in data_for_tests/