Parameters validation

[StrikerRUS]

No description provided.

[fukatani]

Hi @StrikerRUS .
Thank you for your PR.

I merged #25, at the same time, it seems that this branch was also merged unintentionally.
But as you can see, changes to the code by PR are not included in the master branch.
Since I don't know how to reopen this PR, could you reopen this?

Sorry for inconvenience.

If you want to open two PR, I recommend always branch from master, and not work on master branch.

[fukatani]

I recommend:
elif max_leaf <= 0: rather than elif (max_leaf <= 0):
elif not loss in _LOSSES: rather than elif not (loss in _LOSSES):

In future, I apply partial of PEP8 to this repo.

Also we need new test which input type is checked.
Both valid and invalid type should be input.

[StrikerRUS]

Oh, it's my lack of practice with Git, sorry!
Next time I'll open only one PR at the same time, it'll less painful.
I'll add new test and open new PR, sorry again.