- add Loss, Optimizer
- change Trainer & Tester initialization interface: two styles of definition provided
- handle Optimizer construction and loss function definition in a hard manner
- add argparse in task-specific scripts. (seq_labeling.py & text_classify.py)
- seq_labeling.py & text_classify.py work

- move preprocess.py from loader/ to core/
- changes to interface of preprocess: 1. add run method, to run the main processing 2. add cross validation split 3. add return value 4. merge subclasses
- Trainer supports cross validation
- add data as arguments in Trainer.train & Tester.test
- add readme.example.py, to run the example program shown in README.md
- other corresponding changes