Update Backbone from 1.0 to 1.1.

Also update Underscore to 1.6. Includes the development versions of both, which are not included in the build.

Here is Backbone's changelog:
 * Made the return values of Collection’s `set`, `add`, `remove`, and `reset` more useful. Instead of returning `this`, they now return the changed (added, removed or updated) model or list of 
models. (This means they can no longer be chained.)
 * Backbone Views no longer automatically attach options passed to the constructor as `this.options` and Backbone Models no longer attach `url` and `urlRoot` options, but you can do it yourself if you prefer. (But if you extend `wp.Backbone.View`, options will be attached for you.)
 * All `"invalid"` events now pass consistent arguments. First the model in question, then the error object, then options.
 * You are no longer permitted to change the id of your model during `parse`. Use `idAttribute` instead.
 * On the other hand, `parse` is now an excellent place to extract and vivify incoming nested JSON into associated submodels.

See [27171] (next commit) for changes made to WordPress to be compatible with Backbone 1.1.

props gcorne, georgestephanis.
see #26799.

Built from https://develop.svn.wordpress.org/trunk@27170


git-svn-id: https://core.svn.wordpress.org/trunk@27035 1a063a9b-81f0-0310-95a4-ce76da25c4cd