futurize script should preserve function annotations with compatible syntax

[edschofield]

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}

[Uzume]

It would probably make sense to use function decorators to accomplish this, e.g.,

@func_annotations(list)
def _parse(self, filename, dir='.'):
    ...