make message tables prettier

adds styles for tables that display incoming and outgoing messages (e.g., `messagelog` and `httptester` dashboards) so information is easier to read.


adds any column attributes defined in an app's `tables.py` to the corresponding `<td>` elements so app authors may add custom styles to any tables made using `django_tables2` via `django_tables2/bootstrap-tables.html`

here are some before and after pics:

![screen shot 2013-10-07 at 2 11 03 pm](https://f.cloud.github.com/assets/15692/1282913/2ff3edf4-2f7c-11e3-99d1-dd2d0e8094aa.png)
![screen shot 2013-10-07 at 2 11 24 pm](https://f.cloud.github.com/assets/15692/1282911/2fef39a8-2f7c-11e3-962c-ac7c0df4fc8c.png)
![screen shot 2013-10-07 at 2 10 57 pm](https://f.cloud.github.com/assets/15692/1282914/2ff8cd6a-2f7c-11e3-9211-0a573a718a58.png)
![screen shot 2013-10-07 at 2 11 20 pm](https://f.cloud.github.com/assets/15692/1282912/2ff046f4-2f7c-11e3-9aa6-6bdb3b35e86d.png)