Support binary data

Web workers can pass binary data, but this doesn't work in workerpool.

the `stdio` option for child processes was added a bit ago, and allows creation of the ipc channel, which can then be leveraged for sending binary data.

additionally there's the new [`cluster` native module](https://github.com/nodejs/node/blob/master/test/parallel/test-cluster-fork-stdio.js) which automates a lot of this.

i'd love the workerpool API available on top of either one of these approaches so i can have a robust solution that works on both web and node.