Support for "spawn" Multiprocessing

This release supports using "spawn" method of multiprocessing and makes all background processes pickable.

Example:

import multiprocessing as mp
from rmount import RemoteMount
 
if __name__ == "__main__":
    mp.set_start_method("spawn", force=True)
    mount = RemoteMount(config, remote_path, local_path)

Full Changelog: v0.0.4...v0.0.5