Improve collapsed style

Currently if a user was to collapse a table with a thead, the bottom corners have no border radius, but the container does leaving it looking out of place.
![Old look](https://f.cloud.github.com/assets/2552068/2118054/596b593e-90f1-11e3-9311-c9dcdf44e474.png)
This commit adds the thead_collapsed class to the .thead (if present) when collapsed or when set as collapsed from a cookie.
![New look](https://f.cloud.github.com/assets/2552068/2118053/54f1d680-90f1-11e3-95d0-672b31483e30.png)