Improve sibling process exception checking

[Jason Vander Heiden]

Try to remove alive flag in favor or two sentinel object system where termination sentinel in queue indicates end of data and exception sentinel indicates premature sibling process exit.

[Jason Vander Heiden]

I'm closing both both #41 and #15, as the outstanding concerns are part of this issue. Specifically:

1. Better checks for sibling process memory problems.
2. Make sure process termination is working correctly on Windows.