Improve the repr() of angles
Eliminate all Angle() subclasses

Simplify how we protect the user against pulling hours or degrees when
they might not mean to, by having only a single Angle class with a flag
that we set when we instantiate it.
Remove SignedAngle class

Instead we just have an attribute on the angle.  I hope that in the long
run I do not regret loading up angles with these extra little flags.