Gets the tests at least passing with Quantity as an ndarray subclass.  This
required two changes in the existing behavior of the class (both of which could
be worked around, but for now these changes were the easiest way to get things
working):

1. Quantity.value always returns a Python type (such as `int` or `float`) for
   scalar quantities, even if the underlying object is a Numpy scalar array.

2. Quantity instances have a default unit of unscaled_dimensionless (previously
   it was erroneous to instantiate a Quantity without specifying a unit).
Merge pull request #28 from iguananaut/units-refactor-quantity

Gets the tests at least passing with Quantity as an ndarray subclass