ENH: thread pool executor: add `wait_at_exit` option.

Sometimes, it is useful to execute non-essential tasks in
the `EnhancedThreadPoolExecutor` which can be safely killed and
should not prevent the application from exiting, for example
fetching updates information or background indexing of files.
This commit add an optional argument `wait_at_exit` which can
be set to `False` to kill the worker threads abruptly at
interpreter shutdown instead of waiting for them to finish.
Merge pull request #68 from pankajp/thread-executor-do-not-wait-at-exit

ENH: thread pool executor: add `wait_at_exit` option.