Adding archive functionality

Hello, 
I custom wrote an archive sidebar for my site and I thought you might like to include it in your gem (I saw it was on the todo list somewhere).

Feel free to adapt it as needed.

Thanks

Tony Breckner
![screen shot 2013-10-31 at 3 15 08 pm](https://f.cloud.github.com/assets/180383/1443361/cf9fbb76-41e2-11e3-95c6-68fca518b40d.png)