SparseLinear speedup and initialization bugfix

- `stdv = 1./math.sqrt(self.weight:size(2))` is added for consistency with https://github.com/torch/nn/blob/master/Linear.lua#L18.
- Optimized implementations of `SparseLinear:updateGradInput`, `SparseLinear:updateOutput`, and a clearing-grad variant of `SparseLinear:updateParameters` are provided.