* add ignore_type in DataSet.apply

* 添加测试：FieldArray的初始化

* 重构dtype的检测代码，在FieldArray的初始化和append两处，达到更好的代码复用
* 类型检测的责任完全落在FieldArray，DataSet与之配合
测试：
* 整理dtype相关的测试代码
* 给所有tutorial添加测试
其他：
* 完善一个完整的Conll dataset loader
* 升级POS tag model训练脚本

2. FieldArray默认使用AutoPadder, AutoPadder的行为与之前不使用padder是一致的的
3. 为了解决二维padding的问题，引入了EngChar2dPadder用于对character进行padding
4. 增加一份padding的tutorial。

* clean up unused codes

* remove unused codes in metrics.py
* add tests for DataSet
* add tests for FieldArray
* add tests for metrics.py
* fix predictor, add tests for predictor
* fix bucket sampler, add tests for bucket sampler

* In init, detect content type to be Python int, float, or str.
* In append(), check type consistence.
* In init & append(), int will be cast into float if they occur together.
* Map Python type into numpy dtype
* Raise error if type detection fails.

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* remove a lot of out-of-date unit tests, to get testing passed

* remove and fix other unit tests
* add more code comments