Allow a reset on a collection binding to remove views

This is a bit of an edge case, but I had a case where it wasn't practical to add/remove models from the collection as the logic for managing the collection members was on the server. As a result, my resetting the collection wasn't removing views correctly. This patch catches that gotcha, and shouldn't introduce any unnecessary full view resets.

Previously, a reset event on a collection would never remove views. For example:

    users = new Backbone.Collection([{ name: 'nfm' }])
    // Render a view with a collection binding to users
    users.reset([])
    // The exising view for the user nfm will not be removed

This commit ensures that not only is there a view for each model, but also that the number of models has not changed (it can only have decreased if the `collection.every()` call returns true). If it has, we go through the process of resetting with new views.