Adapt to tyxml change for boolean attributes.

See https://github.com/ocsigen/tyxml/pull/30

Also fix this bug, under ocaml 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
```

Just for the viewing pleasure: 
```ocaml
let of_bool = function
   | true -> `True
   | false -> `False
```     
```     
% eliomc -i foo.ml
val of_bool : bool -> [> ` False | ` True ]
 ```
 ```
% ocamlc -i foo.ml
val of_bool : bool -> [> `False | `True ]
 ```
 ```
% ocamlc -i -pp camlp4o foo.ml                                                              
val of_bool : bool -> [> ` False | ` True ]
```

(: