rename: radians2degrees => rad2deg, degrees2radians => deg2rad.

These seem to be fairly industry-standard names.

NumPy:

    http://docs.scipy.org/doc/numpy/reference/generated/numpy.rad2deg.html
    http://docs.scipy.org/doc/numpy/reference/generated/numpy.deg2rad.html

PHP:

    http://www.php.net/manual/en/function.rad2deg.php
    http://www.php.net/manual/en/function.deg2rad.php

Other systems don't seem to ship with functions for this.
Merge pull request #5300 from JuliaLang/sk/rad2deg

rename: radians2degrees => rad2deg, degrees2radians => deg2rad.