Make Markdown tables take 100% width

Tables should take up 100% of the post width - they currently look like this:

![screen shot 2013-12-08 at 1 30 42 am](https://f.cloud.github.com/assets/1193022/1698429/67664e6c-5f4c-11e3-99e9-17b48209d91f.png)