Add links to old versions of Backbone.js documentation

Originally provided from https://github.com/documentcloud/backbone/issues/2086 .

This pull request adds links to old versions of Backbone.js documentation.  

This is handy when you are not yet able to upgrade your Backbone.js version and still want an easy reference to older Backbone.js documentation.

I also added missing links to Diffs for older Backbone.js versions in the changelog.