Work around a circular import

The @cli_tools.load_subcommands() attempts to import subcommand
functions at module load time, leading to a circular import which
results in all subcommands being ignored, since they have to
import the module that contains the decorated function.  We work
around the problem here by renaming notifier.py to notifications.py
and pulling the notification_server() function out into its own
notifier.py, with a comment explaining why the file exists.

This is related to klmitch/cli_tools#2.