pasteurize convert Py3.5 func annotations to Py2.7/3.5 instead of deleting

[mcarans]

pasteurize currently deletes function annotations. Please add a feature to instead convert them to a form supported by 2.7 and 3.5.

Ideally convert to pep-0484 function annotation syntax rather than the Python Future specific format (and add support for pep-0484 syntax to Python Future - see this ticket: #257)