Loky

- Add a `set_start_method` function in `loky.backend.context`. Note
that now, `loky` does not respect the start method set using
`multiprocessing.set_start_method` anymore. It is thus mandatory
to use the `loky` function to have the correct behavior.

- Fix pickling logic in loky. Now the serialization is consistent
between initializer and tasks. Also fixes the logic behind the
environment variable `LOKY_PICKLER`.

- Improve reporting for tasks unpickling errors.

- Fix deadlock when large objects are sent to workers.

- Fix context and `start_method` logic for loky contexts.

- Add a protection against memory-leaks for long running worker
processes: if the memory usage has increased by more than 100 MB
(after a garbage collection), the worker is automatically restarted
before accepting new tasks. This protection is only active when psutil
is installed.

- psutil is now a soft-dependency of loky: it is used to recursively
terminate children processes when available but there is a fallback
for windows and all unices with pgrep installed otherwise.

- Fix win32 failure to kill worker process with taskkill returning 255
- Fix all error at pickle raise PicklingError
- Add missing license file

- Fix bad interaction between `max_workers=None` and `reuse='auto'` (132).
- Add initializer for `get_reusable_executor` (134)

- Fix terminate for nested processes
- Fix support for windows freezed application
- Fix some internal API inconsistencies