Failed attempt at patch for 108

DO NOT merge this - this is just FYI.

I did "the right thing" but I get an obscure error when I try actually to get the item lazily:

    >>> import pint
    >>> pint.Registry
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "pint/__init__.py", line 66, in __getattr__
        self.Registry = self.UnitRegistry()
      File "pint/unit.py", line 417, in __init__
        self.Quantity = build_quantity_class(self, force_ndarray)
      File "pint/unit.py", line 1106, in build_quantity_class
        from .quantity import _Quantity
    ImportError: No module named quantity

Not sure why this is happening, probably not worth continuing...