This patch extends the matrix spec to allow matrix-by-scalar division.

Originally support for `/` was left out to avoid ambiguity for the

matrix-matrix version of `/`, which could either be elementwise or

specified as matrix multiplication M1 * (1/M2).

For the matrix-scalar version, no ambiguity exists; `*` is also

an elementwise operation in that case. Matrix-by-scalar division

is commonly supported by systems including Matlab, Mathematica

or NumPy.