* refine code style
* fix tests
* add a new 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

2. trainer中update_every改为print_every, 因为update_every可能引起optimizer update的误解
3. fieldarray content支持使用np.ndarray初始化

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* remove a lot of out-of-date unit tests, to get testing passed