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* format python code with pep8

* **DO NOT MERGE** deliberately break rules to see what will happen during check

* Revert "**DO NOT MERGE** deliberately break rules to see what will happen during check"

This reverts commit 0db93cd7a43c7efa43a2112ada43d46c6f9115d9.

* fix format in test.py

* add docs for pep-8

* add troubleshooting

* use unittest

* update unittest version

* fix test_engine.py

* fix test_sklearn.py

* default eval_metric by subclass

* add test grid search

* remove verbose_eval

* add gridsearch example for python sklearn

* reset_learning_rate->reset_parameter

* make callbacks public

* add python doc

* add boosting_type to python sklearn

* fix error

current problem:
callback order: user-defined in callback parameter -> reset_learning_rate/print_evaluation/record_evaluation/early_stop
user can't insert a callback between last 4 callbacks

solution:
set order variable:
reset_learning_rate = 10
print_evaluation = 10
record_evaluation = 20
early_stop = 30
user-defined = some int
default: according to index in callback parameter list, = index - len(callbacks) (all < 0)

current callback order:
before iter: user-defined -> reset_learning_rate -> user-defined
after iter: user-defined -> print_evaluation -> user-defined -> record_evaluation -> user-defined -> early_stop -> user-defined

* add feature importances in python; add pandas support

* solve best_iteration issue

1. merge python-package
2. add dump model to json
3. fix bugs
4. clean code with pylint
5. update python examples