Added share entry buttons (Facebook, Twitter, Google+)

I included 3 buttons (12x12 icons + links) that will allow the visitor to share a blog entry on social networks. See screenshots below. I created a new checkbox in the options section of the admin panel to enable/disable the feature. I made everything according to current standards in the Flatpress code. Most blog platforms already have that feature, which is why I went ahead and developed it. 

Blog entry: 
![screenshot from 2014-03-18 22 16 47](https://f.cloud.github.com/assets/6784705/2455952/d88c640e-af0c-11e3-8a7d-ff35f0e0f3dc.png)

Admin panel:
![screenshot from 2014-03-18 22 33 07](https://f.cloud.github.com/assets/6784705/2456055/2274080e-af0f-11e3-80c7-ee9d27b43d8b.png)


I enabled it by default. Feel free to turn it off by default if you want in the fp-default/settings-defaults.php file. 

Thanks. 

Marc