Improve behaviour when posting to a shutdown thread pool

If an executor has a handle_overflow method defined, use that in the case where we post to an executor which is not running. If no handle_overflow method is defined (e.g. for the single-thread executor), preserve the current behaviour of returning false.

Fixes Github issue #192.
Make tests tolerate varied shutdown behaviors
Merge pull request #199 from rkday/post_to_shutdown

Improve behaviour when posting to a shutdown thread pool