Fixed Headers directives : fix thead bugs

It's said that one drawing is better than a thousands words... So this PR fix this:

![fixed-headers](https://cloud.githubusercontent.com/assets/116530/4272296/f7ae4e94-3ce0-11e4-9fa1-acd7e2b88943.png)


Should be merged after https://github.com/Evaneos/ev-fdm/pull/24


![r3lzzkk](https://cloud.githubusercontent.com/assets/116530/4272173/db979b62-3cdf-11e4-8565-b8e68b775e93.gif)