Adapt to tyxml change for boolean attributes.

Also, fix this monstruosity with camlp4 3.12 :
```
File "src/ew_editable.eliom", line 32, characters 32-37:
Error: This expression has type [> ` True ]
       but an expression was expected of type
         [< `False | `True ] Eliom_content.Html5.F.wrap
       The second variant type does not allow tag(s) ` True
```
Merge pull request #5 from ocsigen/bool_attrib

Adapt to tyxml change for boolean attributes.