Generator object has no attribute start_pos (added a failing test)

This is a bug I noticed when I writing #176.

Running py.test raises this error:
```tb
test/test_api_classes.py:50: in <module>
>   @pytest.mark.parametrize('definition', make_definitions())
test/test_api_classes.py:38: in make_definitions
>       definitions += script.definition()
jedi/api_classes.py:44: in wrapper
>       result = func(*args, **kwds)
jedi/api.py:274: in definition
>                   if not isinstance(s, imports.ImportPath._GlobalNamespace)])
jedi/api_classes.py:418: in __init__
>       super(Definition, self).__init__(definition, definition.start_pos)
E       AttributeError: 'Generator' object has no attribute 'start_pos'
```