Remove Quantity.copy() as ndarray version is fine; add tests for order

For `MaskedArray`, I needed `Quantity.copy` to take an `order` argument. After implementing it, however, I realised that `copy` does not have to be overwritten at all (at least for the versions of python/numpy I tested). So, this PR removes it, but adds test that check that the `order` argument works. Now see if travis agrees that this is OK for all versions...