Added BaseController and SiteView

Chaplin supports use of regions which abstract the process of defining
and referencing DOM elements. For example, instead of adding a view to
the `#main` element, we can add it to the `content` region.

The new BaseController is responsible for rendering a SiteView in its
beforeAction.

The SiteView renders a template that sets up the views for the site and
then registers the `content` region as a reference to `#content`. (Note
that we can change this in the future without having to change the other
controllers.)

This commit also updates the WelcomeController to use the new
BaseController and `content` region.
Added tests for BaseController
Initial commit of Todos UI
Moved models/todos to models/todo
Use new BaseController and BaseCollectionView
Removed window.collection from TodosController
Use 'content' region in TodosController
Added 'todo' className to TodoView
Added tests for TodosController and TodosView
Added a link to the todos on the welcome view