add optional process_creation_flags argument to _install_update_win()

[dennisvang]

The optional process_creation_flags argument allows users to override the process creation flags used for the subprocess call that runs the windows batch script that does the actual installation (i.e. moving files into place).

The process_creation_flags argument accepts subprocess Windows constants.

The default value remains subprocess.CREATE_NEW_CONSOLE, as before.

Usage example:

client.download_and_apply_update(..., process_creation_flags=subprocess.CREATE_NO_WINDOW)

fixes #81