Should set numOfWorkers to 0 automatically when Blob isn't available

[ericblade]

In the present version of the code, numOfWorkers > 0 fails in node due to (at the very least) Blob not being available. While Node probably can support the workers system now (probably with some changes), it would need to be fairly significantly overhauled to use Buffer instead of Blob, and then that would create browser issues, so we'd need to shim some sort of Buffer/Blob compatibility thing, or just re-do the worker part.

THEREFORE, I believe we should have the code basically check if (typeof Blob) { config.numOfWorkers = 0 } instead of allowing you to attempt to use it where it can't work, and getting failures that don't make sense.