add a turtle.getDistance(anotherTurtle) method

This makes it easier to compute the distance between two turtles.
It's a simple overload that passes the other turtle's x and y
coordinates to the numeric getDistance method.
fix wrapping/typesetting on changelog

In order to make the changelog easier to email, I originally fixed
the width at 72 characters. But since my editor's margin guide is
set to 78 characters, I kept wrapping new entries at 72 characters.
This cuts the overlength entries back down to 72, and also fixes
lines with "orphan" words.