Interface change: Make theano.tensor.signal.conv2d(2d,2d) output 2d answer.

Right now, it output 3d with the leading dimensions of size 1.

Don't merge before June 7th.

NEWS.txt:
Interface change.
- theano.tensor.signal.conv2d(2d,2d) output 2d answer. (Frederic B., reported by Alexander Izvorski)