Use is_callable() to validate notification_callback

This allows the full gamut of callable formats to be used as notification
callbacks - eg, array( $my_object, 'my_callback' )

Fixes #4860

Props sbrajesh

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7173 cdf35c40-ae34-48e0-9cc9-0c9da1808c22