READY: Gravatar

1. Adds gravatar virtual getter to Players schema that builds a gravatar url.
2. Calls toJSON({virtuals: true}) on all Player queries to ensure that gravatars are returned.
3. Adds gravatar images to all pages.
4. Adds badge coloring based on who one each individual game.

Maybe we need a way to tell people "Hey, here's how you get your image on the site..." Ideas? Here are mine:
* Use a cookie and dialog
* Add a tooltip on all images so on hover people will see the instructions
* Add a button (somewhere?) with a popover and instructions
* Add a FAQ page

Or, of course, there's always email and/or word of mouth.