Kronecker product needs a SparseMatrix implementation

The current implementation of the Kronecker product #704 silently changes sparse matrices into dense matrices.

Example:

```
v1 = math.matrix([0, 2, 3, 0, 0], 'sparse')
v2 = math.matrix([0, 1, 0 -1], 'sparse')
v = math.kron(v1, v2)
```

Returns `v`, which is a `DenseMatrix` even though `v1` and `v2` are`SparseMatrix`.
I would expect `SparseMatrix` instead.

Is it intended behavior?