Add 'dynamic favicon' setting

This adds a setting called 'dynamic favicon' that puts incoming message counts in the favicon! (just like we already do in the site title)

Why on earth would we want to do this? Because it's awesome and lots of people like to run Chrome with pinned tabs. Here's how the default Discourse dynamic icon looks sitting next to all my Gmails:

![all-tabs](https://f.cloud.github.com/assets/65323/626800/9da5224c-cfd1-11e2-84e1-ec7a557824e9.png)

And here's how amazing meta will be with this setting enabled :smiley: 

![zero-count](https://f.cloud.github.com/assets/65323/626781/0ec36fe8-cfd1-11e2-866e-360c3beb4ad9.png)

![ten-count](https://f.cloud.github.com/assets/65323/626782/10879f8e-cfd1-11e2-8151-f7f4fe7eea84.png)