Remove hard-coded "add_user" capability in ComponentAbstract

[griffbrad]

When calling add_object_page() and add_submenu_page() in \Dewdrop\Admin\ComponentAbstract, we currently hard-code the "add_users" capability. This should be replaced by a class property that can be manipulated by Component classes in their init() methods.

Submenu pages should be allowed to use capabilities that differ from the component's global capability value, but they should default to using that global value.

See http://codex.wordpress.org/Roles_and_Capabilities for more information about capabilities in WordPress.

Once you have this in place, talk to Brad about how best to test this functionality.