Closure: create element in lower case only

When using oclosure with Eliom, the script generated the iframe element incorrectly. 
The script after which retrieved the iframe and perform further parsing then failed accordingly.

![screen shot 2013-07-31 at 19 11 22](https://f.cloud.github.com/assets/1419998/885705/a39c33b8-f9d7-11e2-8e56-6b72d23b424f.png)

By forcing all the element to be created in lower case solved the problem.

![screen shot 2013-07-31 at 19 02 26](https://f.cloud.github.com/assets/1419998/885704/9a11cbf0-f9d7-11e2-8014-3ddc9945566f.png)

The issue also happen in other element such as div.