Save Twitter avatar to user meta & show with get_avatar

Uses the first Twitter API call to retrieve the avatar URL, then saves it to the user meta "social_connect_twitter_avatar" which is later used when displaying the user avatar. Unfortunately it won't change when the Twitter user changes the avatar, but that's how the 1.1 API works. You'd have to make a new API call to retrieve updated account information.
Please follow WordPress coding standards for new code (although old parts of the plugin code don't follow it).