Yolean Backbone

To make our Backbone based code more maintainable we need:

1. A .mixin as complement to .extend. Subclassing works for specialization but not for composition.
2. A separation between the jQuery and browser dependent View/Router/sync stuff and the data structures Model/Controller. The latter is used server side.
3. We don't need a default .sync at all.
4. Ways to use the same backbone instance across modules, or alternatively override the default _isModel in the upcoming Backbone release. Basically anything with a .attributes can be considered a model and shouldn't be wrapped at .add.
5. Complement Collection .filter and .where with .subset that returns a connected Collection

This implies that the lib must extend default functionality of backbone. That goes for the re-export in bullet (4). It should not however modify any existing functionality, but it can export mixins that do so.