angle_utilities - fix to enable padding of negative angles

In response to issue <a href="https://github.com/astropy/astropy/issues/684">#684</a>.

Angle.format was not padding negative angles, despite passing in pad=True. The problem was in angle_utilities.degrees_to_string and hours_to_string - they both needed a small change to account for the extra character when a negative sign is present. Also added some test assertions to ensure the fix is working correctly.