Update Backbone.localStorage

A lot of work has been done on the localStorage adapter for Backbone. The updates have been flowing in this repository: http://github.com/jeromegn/backbone.localStorage

The version currently being used in the todos example is very old and it does not reflect the current state of the Backbone.localStorage adapter.

Although functionalities don't differ much, the latest version has performance and stability improvements (namely, it has tests). It's also actively being maintained.

I've updated the docs to reflect the changes.