Fixed inconsistent renaming of *args and **kwargs.

minipy was renaming *args and **kwargs only at Call nodes, when it
should be renaming them (just like any other non-keyword argument)
at FunctionDef nodes too. This behavior make simple programs crash.

This:
```python
def x(arg, *args, **kwargs):
    print arg
    print args
    print kwargs
```
was being minified to this:
```python
def x(a,*args,**kwargs):print a;print b;print c
```