Clean up cruft; move common fn to utils

Many of the audience finders were not implemented
and had empty execute functions.  This was really
misleading.  Remove these files so they don't confuse
others since they don't serve any purpose.

Move a common function that was used to look up all
users with given ids into a utility module.
Have the audience finders use this utility function.
Clean util function

Add documentation, pass err correctly to the callback,
and make it part of the module instead of a standalone
function.