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 is an array of the different page types that are presented to the user after they have selected this theme. In this case there is only one page type available, the "Colors Demo" page. The object that follows has properties that apply to the page (`displayName` is what is shown to the user as the name of that page, the thumbnailUrl tells where to fetch the image to show as the thumbnail image).

The `sections` part of the page object shows the sections that are added by default when the user starts editing this page, and the order they will be added. In this case there is a 'hero' section, with a headline of 'Color Monster',followed by a 'color-boxes' section with a headling of 'Calculated Colors'