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

Switch from using add_options_page(), which is single-site specific,
and use add_submenu_page().

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.

Addresses #2.
Merge pull request #4 from r-a-y/multisite-menu

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