[python-package] add plot metrics

[wxchan]

#171

[msftclas]

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[wxchan]

an example in examples/python-guide/plot_example.py:

[guolinke]

@wxchan Did this finish?

[wxchan]

@guolinke wait a minute.

[wxchan]

@guolinke I change one misleading comment. Now finished. You can wait till the check passes.