Replace 3 character hex color codes with 6 character codes.

Replace shorthand three character hexadecimal color codes with full six character hexadecimal codes as recommended in the following sources:

* Zurb's [Responsive Email Gotchas to Avoid](http://zurb.com/university/library/23)
* Litmus' [The Best Way to Code Background Colors for HTML Email](https://litmus.com/blog/background-colors-html-email)
* Campaign Monitor's [Email Design Guidelines](http://www.campaignmonitor.com/resources/will-it-work/guidelines/)
* Campaign Monitor's [Outlook 2013 doesn’t respect height in empty table cells](http://www.campaignmonitor.com/blog/post/3795/outlook-2013-says-no-to-empty-table-cells)

*Litmus Tests*
* [Basic](https://litmus.com/pub/9fd5b02)
* [Hero](https://litmus.com/pub/eb24004)
* [Hero Sidebar](https://litmus.com/pub/77e099d)
* [Sidebar](https://litmus.com/pub/e512618)