Convert existing notifications to use user-locators

Replace implementation specific code with generalized user-locator
implementations that can be re-used by more notifications in other
extensions.

This patch adjusts the `user-locators` notification parameter to allow
arrays which facilitate passing options to the locator.

Previously(still works):

    'user-locators' => array( 'foo', 'bar' ),

New functionality:

    'user-locators' => array(
	'foo',
	array( 'bar', 42 )
    )

In the second example the callback specified by `bar` will receive
an EchoEvent as the first argument and array( 42 ) as its second
argument.

Change-Id: I7305279bc91d1e40e7054e2fd42a819a35526b82