Hide Skype for users that are not logged in

Keep Skype accounts safe from internet bots.
This is in response to some Skype spam the other year, caused by
putting my Skype account on my Python course account. This caused
a week full of spam until I removed Skype from my Python course user.
Merge pull request #51 from rosti/master

Keep registered users Skype accounts safe from internet bots.