Make FilteredList's work more like computes

[akagomez]

Computes don't setup bindings on dependent properties until they're bound:

var firstName = can.compute('Chris');
var lastName = can.compute('Gomez'); 
var fullName = can.compute(function () {
  return firstName() + ' ' + lastName(); 
}); // Nothing is done
fullName.bind('change', function () {
  // Do something
}); // Binds to "change" events on `firstName` and `lastName` 

We should be able to do something similar with FilteredList's:

var source = new can.List(['a', 'b', 'c']); 
var filtered = source.dFilter(function (letter) {
  return letter === 'b';
}); // An empty FilteredList is returned

filtered.attr(0); // Iterates `source` until the first element that passes the predicate is found

filtered.bind('add', function () {
  // Do something
}); // Binds to "add", "remove" and creates computes for each element in `source`.