Use native BP options API where applicable.

This fixes an issue on multisite when BuddyPress is installed on a
secondary site (when BP_ROOT_BLOG != 1) and the proper options were not
being fetched / set.
In admin settings, register settings on 'admin_init'.

Registering settings on the 'admin_init' hook allows us to do checks for
other plugins, which should have initialized by this point.
Add CBOX option to set the Forum tab as the default tab on a group's page.

Fixes #74.
Fix notice in admin settings loader.