Fix for column definition on tablets

# What
Fixes something that can be seen in #69: the 2 main columns aren't defined very well for small devices (tablets >768px).
Also, the categories in the sidebar had a column definition, which forced long categories to use 2 rows, while there was plenty of room, so I removed that column definition.

This change does however not fix the menu being rendered on 2 columns.

# How it looks
This is what it currently looks like:
![screen shot 2014-10-11 at 18 39 30](https://cloud.githubusercontent.com/assets/106844/4603474/8d35ea4a-516c-11e4-98ce-71c46f1396d0.png)

This is what it looks like after these changes:
![screen shot 2014-10-11 at 19 31 51](https://cloud.githubusercontent.com/assets/106844/4603475/95d482f6-516c-11e4-854b-9410b0210aab.png)