Fix reraise on Python 2 and Python 3

Python 2: be a real function, rather than a nested one
Python 3: need to check that value was an Exception
Merge branch 'fix-reraise' of https://github.com/jtratner/python-future into jtratner-fix-reraise
Rename future.utils.reraise to future.utils.raise_
Add raise_with_traceback(); describe this and raise_ in changelog

Also add an alias of reraise to raise_ for backward compatibility