Automatically determine the number of CPU Cores

[eholk]

Instead of getting parallelism via RUST_THREADS, we should, by default, find the number of CPU cores and use that. We should keep RUST_THREADS around though, in case we want to run sequentially in some cases.

This discussion seems to list how to do it on each of the platforms we support: http://stackoverflow.com/questions/150355/programmatically-find-the-number-of-cores-on-a-machine