convnfix

Hi,

this is a small change to quickly overcome the convn() problem in Octave. It is pretty fast with each epoch of the test_example_CNN test case taking arround 450 seconds(batchsize==250).
I introduced a new function called conv_valid that uses conv2 to compute convn(), and explicitly make the decision on which function to call in the cnnbp code so once the problem in Octave is solved the update will be simple.

Alberto.