In multisite, fix menu not showing up under the "Settings" menu

Hi Paul,

This pull request addresses issue #2 with the settings page not showing up in the Network Admin area.

Specifically, this patch switches from using `add_options_page()`, which is single-site specific, to use `add_submenu_page()`.  This is because multisite uses a different parent slug than single site, so we need to
toggle the slug depending if we're in the network admin area.

Let me know if you have any Qs!