Stop raising AttributeError when wrapping funcs

Before Python 3.2, functools.wraps (used in UnitRegistry.wraps) raised
an AttributeError when wrapping functions without all of the attributes
in functools.WRAPPER_ASSIGNMENTS and functools.WRAPPER_UPDATES. In
Python 3.2, this limitation was removed. This commit removes the
limitation in older versions of Python, as well.
Merge branch 'backport-wraps' of git://github.com/jturner314/pint into jturner314-backport-wraps
Updated CHANGES