randmatmul implemented by calling out to libblas, just like
the other benchmarks. Note that about 90% of the time for the
rand_mat_mul benchmark is now taken up by random matrix generation.

IMPORTANT: This change makes the Javascript benchmark run only in
node.js. The 'ffi' and 'ref' npm modules need to be installed. libblas
must also be available as a dynamically loadable library. The npm modules
are installed as -

    npm install ffi
    npm install ref