Class_<XXX> can define methods

The problem laid on aspects. When call_before('method_name') was
triggered before_filters object was empty while we were looking for:

```javascript
before_filters = {
  greet: []
  .
  .
  .
}
```

The following firebug output was the clue.

>>> obj.greet.toSource()

"(function greet(){ this.call_before('greet');
 return 'hello. I am ' + this.name ;
this.call_after('greet');
})"

>>> obj.call_before.toSource()

"(function (fn_name){
  for (var i=0; i<this.before_filters[fn_name].length; i++)
    this.before_filters[fn_name][i]();
})"

>>> obj.call_after.toSource()

"(function (fn_name){
    for (var i=0; i<this.after_filters[fn_name].length; i++)
       this.after_filters[fn_name][i]();
})"

>>> obj.before_filters
Object { tainted=false, _trust=true, _FROZEN=false, más...}
>>> obj.before_filters['greet']
undefined"
})
})"