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.
The `{{css-color}}` helper should be relatively self-explanatory. It can make use of any of the three color properties from the theme (`lightColor`, `midColor` and `darkColor`), and it can be given a 0-100 value to either lighten or darken it.

See `app/helpers/css-color.js` for more info on how this helper works.