Place options page under 'Settings' in stead of 'Plugins' menu

The [Administration menu](http://codex.wordpress.org/Adding_Administration_Menus#Determining_Location_for_New_Menus) Codex article suggests that a 'Plugins' menu item
> Displays controls dealing with plugin management, not configuration options for a plugin itself.

Adding the options page under 'Settings' is probably a better choice. Fixes #60.