Adding image to user %Info (#34)

Pretty much all other Ueberauth strategies use image: for purposes of the user avatar. I suggest doing this same with the GitHub strategy, so as to avoid needing to customize the templating for this strategy.

As an example see: these other strategies

LINE 111 in https://github.com/ueberauth/ueberauth_facebook/blob/master/lib/ueberauth/strategy/facebook.ex

LINE 109 in https://github.com/fajarmf/ueberauth_linkedin/blob/master/lib/ueberauth/strategy/linkedin.ex

LINE 76 https://github.com/ueberauth/ueberauth_twitter/blob/master/lib/ueberauth/strategy/twitter.ex

lib/ueberauth/strategy/github.ex

@@ -172,6 +172,7 @@ defmodule Ueberauth.Strategy.Github do
       nickname: user["login"],
       email: user["email"] || Enum.find(user["emails"] || [], &(&1["primary"]))["email"],
       location: user["location"],
+      image: user["avatar_url"],      
       urls: %{
         followers_url: user["followers_url"],
         avatar_url: user["avatar_url"],