Fix gradcheck and add softmax backward

[wkcn]

1. Fix the bug in gradcheck
2. Add Softmax Backward
3. Add the testing requirement nose and coveralls into setup.py

NOTICE: it is not suitable to use the function gradcheck to check the gradient of Softmax Activation.

[coveralls]

Pull Request Test Coverage Report for Build 812

• 26 of 34 (76.47%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 85.526%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
opzoo/Softmax/Softmax.py	0	8	0.0%
<!--	Total:	26	34	76.47%	-->

Totals
Change from base Build 810:	-0.2%
Covered Lines:	1755
Relevant Lines:	2052

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💛 - Coveralls