Table: Fix cell width for cells spanning multiple columns

**Before**
![bildschirmfoto 2014-01-16 um 11 14 06](https://f.cloud.github.com/assets/7469/1929390/10046ae0-7e97-11e3-898d-a984362c66dc.png)
---
**After**
![bildschirmfoto 2014-01-16 um 11 14 21](https://f.cloud.github.com/assets/7469/1929392/15d4dde2-7e97-11e3-8fe2-7fc0d4052cf2.png)
---

The source of the problem is that defined width of a column does not include the padding (`\tabcolsep`), which is applied to the left and the right border for each cell.
That means, if a cell is defined with `0.5\tablewidth` the consumed with is `0.5\tablewidth + 2\tabcolsep`.

The problem then occurs because `multicolumn` does not compensate the `\tabcolsep` in between when spanning multiple columns.
This fix does manually add this padding (`2 * (colspan-1) * \tabcolsep`) to the width for colspan cells.