fix bind('list.length') - make length event bubble

[iamnoah]

See http://jsbin.com/ikewoy/4/edit

Trying to bind the length of a nested list does not work because length does not bubble. I don't know why length is special, but this patch makes it behave like any other attribute.

[justinbmeyer]

Length is kinda a special event, mostly used to listen to add and remove without having to bind to both. Can you let me know how listening to this on a nested list makes your life easier? If you do, we'll pull it. Thanks!

[iamnoah]

We use it for templated binding in our controls for one:

"{model} items.length": function() {
   this.element.find('.itemCount').html(...);
}

[dispatchrabbi]

You should be able to accomplish that just using:

"{items} length": function() {
    this.element.find('.itemCount').html(...);
}

If you can find a case where this kind of solution doesn't help you, rig up a test case and we'll take it from there.

[daffl]

This unfortunately doesn't seem to solve #267. Will investigate more in that issue but closing this PR.