fix clip bug.
Added tests for the other way around as well.
fixed the unit tests.
Added the comments.
Merge pull request #2225 from caglar/fix_grad_clip

Fixes #633
missing c_set_tensor4d in GpuDnnConvBase
Merge pull request #2241 from ynd/dnn_conv_fix

missing c_set_tensor4d in GpuDnnConvBase
pep8
add docstring and swap parameter order to be the same as make_node
doc cudnn stuff
Now by default, if cuDNN is available, always use it.
better doc
make dnn detect if the library can be loaded.
Better error message when Theano isn't able to use cuDNN