display the share count

Makes a call to the twitter and facebook api's to work out how many shares have been made.

On wider screens it will do a little animation to increment up the count. It will only show the count if there are 4 or more. This could be changed to show if there are any shares at all—interested on other's thoughts there.

![screen shot 2014-12-10 at 10 22 09 am](https://cloud.githubusercontent.com/assets/1239550/5369222/860d5ac4-8064-11e4-8894-c5173161f51c.png)

big thanks to @jpillora and the Guardian team for there code

closes #1004