- 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

- apply DataSet in Predictor; remove sub-predictors; add "task" argument to specify which task to predict, as how Trainer/Tester did.
- remove Action class
- add helper function for DataSet, to create DataSet easily
- more code comments
- clean up unnecessary codes
- add unit tests for Batch, Predictor, Preprocessor, Trainer, Tester