Fix table margin.

This PR should fix the table layout (tables' margin was set to -0.5em which caused them to be overflowed by the sidebar. See screenshot).
![screen shot 2014-08-11 at 11 46 11](https://cloud.githubusercontent.com/assets/1692189/3874664/bd133036-2144-11e4-8921-7b00ee40e5cf.png)