Add `is_unity` as a method on Units, rather than an external function.  This has performance benefits.
Handle the dtype conversion within _validate_value
Raise an exception when raising a Quantity to a non-scalar power.
Make divmod(Quantity, Unit) work
Merge pull request #36 from mdboom/unit/quantity-subclass-of-ndarray-extras

quantity subclass of ndarray extras
Implemented a few further suggestions by @mdboom