Handle non-functional listeners to
Remove the groupWatch stuff

In reality, $computed covers this use case better than than $groupWatch
does. We have taken their idea of triggering off a single incrementing
variable (rather than off any one of the dependencies). Other than that:
just map over the expressions you want, if you want to:

var values = ["a","b","c"].map(function(expr) {
  $scope.eval(expr);
});

We may provide a helper to that more easily in future, but not for now.
Update docs to remove group stuff and add some debug info
Add a link to debug docs in the README