Fix meetings overlap

Actually, we got this:
![chevauchement](https://cloud.githubusercontent.com/assets/224733/4517994/10f70f56-4c6a-11e4-9b34-bcbfa654c202.png)

Simply removing td elements fixed height fixes the issue. I first try to use min-hegiht instead of height on those elements, but min-height is not supported on table elements.
The solution is here to create a div into empty tds, and apply a min-height to those new elements. I'm not fan of empty elements in the HTML, but I really do not found any other working solution.