make this widget includable by a custom modules

@antonylesuisse 
Make the wms widget includable in custom modules.

Don't use IIFE for openerp's client module declaration.
The correct way is to declare it in a function assigned to the scope name of the module:

```javascript
openerp.stock = function(instance) {
   // module code
};
```
Note: the old code was a bit wierd.