Post Aside Shortcode

Looks like this:

![screenshot 2014-10-22 15 13 37](https://cloud.githubusercontent.com/assets/1757914/4742504/913aaa52-5a1f-11e4-9767-2ca51133f725.png)

We may want to revise this at some point, since having three columns is a little cramped, but it fulfilled a request and may be useful in the future.

I opted for a shortcode implementation over a custom field for display in the main sidebar because it seems likely that adding an additional sidebar custom field below the content would encourage people to use the same old bad practices as on the old site with custom field sidebars.