Multiple Optional objects

Having more then one Optional Object fails parsing the parameters.

```
@tag(register, [Constant('as'), Name(), Optional([Constant('for'), Variable()]), Optional([Constant('count'), Variable()])])
def get_tagcloud(context, asvar, forvar=None, countvar=None):
```

I think the problem lies inside the **call** method of the Parser Class, which assumes that all parameters are present in the order they are defined.