Backport inspect.unwrap from Python 3.4?

[ncoghlan]

http://bugs.python.org/issue23764 highlighted that one of the useful features inspect.signature offers over the old APIs is correctly following wrapper chains.

Python 3.4 split that feature out into an independently usable component: https://docs.python.org/3/library/inspect.html#inspect.unwrap

There are some other features potentially worth backporting from 3.4 (like the inspect.signature based inspect.getargspec and inspect.getfullargspec), so a full rebase may even be worthwhile.