[MRG] Early stop

[haifeng-jin]

This pull request resolves #288 .

The pull request is ready to be merged.

The details of the pull request:

[coveralls]

Pull Request Test Coverage Report for Build 847

• 41 of 47 (87.23%) changed or added relevant lines in 3 files are covered.
• 8 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.5%) to 94.425%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
autokeras/nn/model_trainer.py	39	45	86.67%

Files with Coverage Reduction	New Missed Lines	%
autokeras/nn/model_trainer.py	1	92.97%
autokeras/cnn_module.py	1	94.92%
autokeras/net_transformer.py	1	97.53%
autokeras/search.py	1	97.28%
autokeras/bayesian.py	4	95.02%

Totals
Change from base Build 840:	-0.5%
Covered Lines:	2151
Relevant Lines:	2278

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💛 - Coveralls