futurize script should preserve function annotations with compatible syntax

Spawned from issue #2.

Running `futurize --from3` should convert Python 3 function annotations such as::

```
def _parse(self, filename: str, dir='.') -> list:
    ...
```

into the following Python 3 / Python 2 compatible code:

```
def _parse(self, filename, dir='.'):
    ...
_parse.__annotations__ = {'filename': str, 'return': list}
```
