Add initial Backbone guidelines

Conventions in the Backbone.js community:

http://ricostacruz.com/backbone-patterns/#conventions
https://learn.thoughtbot.com/products/1-backbone-js-on-rails
https://learn.thoughtbot.com/workshops/22-hands-on-backbone-js-on-rails

The common alternative naming convention `App.Models.Photo` /
`App.Collections.Photos` / `App.Views.Photo` is unnecessary in Asynchronous
Module Definition-style projects using a library like RequireJS.

http://requirejs.org/docs/whyamd.html
Add "Accepting a GitHub Pull Request" protocol

The only potentially controversial part of this protocol is the `co-pr` alias.

In general, we've avoided using aliases and non-standard scripts when describing
our protocol. For example, in the [Git Protocol], commands such as `git fetch
origin` and `git rebase -i origin/master` are spelled out, even though just
about everybody uses an alias for these steps.

[Git Protocol]: https://github.com/thoughtbot/guides/tree/master/protocol/git

My argument for showing the alias here is that I don't think it's very pragmatic
to have the pull request number repeated three times in the guide:

    git fetch origin pull/123/head:pr/123
    git checkout pr/123

As programmers, we're all likely to use an alias for this. I think the right
balance is to define the alias in the guide, and then use it.