Handle SIGCHLD properly for multiprocessing

This commit should fix #101 by enabling a special SIGCHLD
handler for when multiprocessing is in use.  The handler
simply calls `multiprocessing.active_children()` (which in
turn calls `_cleanup()`) upon receiving a SIGCHLD.  Now,
the `fallback_SIGCHLD` is only called when `multiprocessing`
is not in use.  See also #95.