Break down todo tutorial into smaller steps

[unlikenesses]

Split chapter 4 / exercise 1 (the todo exercise) into smaller, progressive steps. Introduce elements in the todo app gradually rather than in one go. Rewrite text where necessary to make narrative flow between the new steps. Add several links to the Backbone.js documentation to clarify some points.

Text mostly from http://unlikenesses.github.io/2015/12/16/backbone-todo-tutorial-part-one/ and http://unlikenesses.github.io/2015/12/17/backbone-todo-tutorial-part-two/.

[addyosmani]

@unlikenesses Thanks for this work! I would love to work with you on landing it. Some minor changes in the PR queue were merged to master before I got to this one and it's introduced need to rebase against master. Do you think you'll have time to do this? Alternatively, I can work against your PR to do so but wanted to check if you were interested in following up.

[unlikenesses]

@addyosmani Great! This is my first PR so I want to proceed, erm, carefully... If you or anyone else can confirm what precisely I need to do I'll go ahead and do it. Whatever's easiest for you.