added social media icons to invite friends

Just some buttons to social media. It makes easier to spread the link.
I used Fatcow icons. They are like "an extension" of famfamfam icons. They can be used under the same rule.
http://www.fatcow.com/free-icons
Note: I also used this icons to make achievements medals.
![facebook](https://cloud.githubusercontent.com/assets/6611118/3652671/320beb04-1140-11e4-9f91-f98cff3bd513.png) facebook.png
![twitter_logo](https://cloud.githubusercontent.com/assets/6611118/3652673/337f5aca-1140-11e4-9274-8f54e77ab81a.png) twitter_logo.png
![google_plus](https://cloud.githubusercontent.com/assets/6611118/3652674/34a1efe4-1140-11e4-91df-cc67f482d843.png) google_plus.png

![new-3](https://cloud.githubusercontent.com/assets/6611118/3652705/c8fb30b0-1140-11e4-8d5e-2fd76cdecef5.png)