Fix issue #157: Display negative imaginary component

[Astronomy487]

This PR fixes #157. The original implementation of Display checks what sign is needed between the real and imaginary components by comparing the imaginary component to T::zero(). This fails for negative-zero, which should be treated as negative but not less than zero, producing 2+-0i when it should produce 2-0i.

This fix replaces a comparison against T::zero() with the is_negative method. This fix replaces the trait bound of PartialOrd with a trait bound of Signed. Technically can break people's custom types that implement PartialOrd but not Signed. Added an appropriate test to test_string_formatting.