Verify grad speedup

Ccw 4483 indent fix

ones_like and zeros_like dtype parameter is documented

Useless alloc

Don't use DebugMode for the numerical gradient in verify_grad.

Exclude optimization to keep old test working

Set cuda/test_dnn to run conv references in fast run

Unbreak travis.

* added function to look for working march

* update to reflect comments

* correction

* flake8

* raising ValueError if no working flag found

* updated according to comments

* import six

* importing correctly

* respecting flake8

* whitespace removed

* addressing comments from nouiz

* this might be it

* minor edits

* pep8

ccw3398