Allow Notification callbacks to be methods and not functions only

[sbrajesh]

Hi,
currently, the notification rendering mechanism in BuddyPress needs the notification_callback to be a function and does not render it if we register a method on some object.

We need to change function_exists to is_callable.

here is a patch attached against r6829

[boonebgorges]

In 7173:

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