Use explicit indexing to change selected status of collection models

The reason for this change is that integer values in the parsed
collection were found to return undefined from the findWhere call
because of the evaluation used by Backbone here:

    http://backbonejs.org/docs/backbone.html#section-107

For that reason, we avoid any lookups(and thus comparison nuances) and
just use explicit indexes into the collection as the DOM order and
collection order are known to be the same.