Add color monster theme

This PR (and its single commit) contains everything that is necessary to add a new theme (and nothing else)

There is a video demo here: http://screencast.com/t/ezDriv4P97Rx

  * Adds `app/themes/<theme-name>.js`, the file that defines the theme's structure
  * Adds `templates/themes/<theme-name>/<section-name>.hbs`, one template file for each of the theme's sections that can be added by a user
    * this theme adds `color-boxes.hbs` and `hero.hbs` because those are the two sections it uses, as defined in the `app/themes/color-monster.js` file
  *  Adds `templates/themes/<theme-name>/styles.hbs`, the file that is used to define specific styles for this theme, and shows the use of the `{{css-color}}` helper in it to show how to use dynamic/derivative styles based on theme variables
  * Updates app/themes/index.js` to add this theme to the list of possible themes

See in-line comments below for more detail.
This lists the sections that the user is allowed to add to the page as they are editing it. This is what makes it so there is an "add hero" and "add color-boxes" button in the editor when editing this page.