Make `plugins` branch

[afeld]

We don't have to spend time explaining it in the book, but would be cool to have a branch where we fully leverage community-built plugins and to show how much of the cruft could be cut out. We can link to the branch/compare view from the book/README, just as a demonstration.

Plugins to be included, for starters:

• LayoutManager
• Backbone-Super
• something for associations/relations

[afeld]

Haha jinx #45

[tbranyen]

Maybe stickit as well for binding.

[afeld]

From @tbranyen in #45: backbone-boilerplate, and maybe "TodoMVC route where different options are included?