rel nofollow to mailto icon

This PR adds rel="nofollow" to the mailto link in com_content. This is done in order to stop search engines from following the link.
 
![skaermbillede 2014-10-17 kl 17 17 00](https://cloud.githubusercontent.com/assets/1738811/4681148/af9a98da-5610-11e4-9f5c-3f98ed38874a.png)