Add info in the doc of sparse dot ops.
Expend doc for softmax.
Add assert to detect error earlier.
Add a test for the softmax with the new keep_dims parameter.
remove import not used.
More debug print for device malloc/free.
In perform, use vectorised version of the code.
make nosetests more verbose to help fix the no output for 10minutes in travis.

If it happen frequently on the same test, we could modify this test to be more verbose or faster.
Precheck the error BEFORE the check for memory available.
Merge pull request #1536 from nouiz/doc

Mixed small stuff