Get ready for release 5.0.1

Overview

Mostly a release to fix a Python packaging problem.

• format and do_format methods were removed from the interface of
  BaseElement, becoming non-member functions.
• The class BoxElement was introduced as a base for boxing elements.

New Builtin

• 'Inverse Gudermannian'.

Documentation

Hyperbolic functions were split off form trigonometry and exponential functions. More url links were added.

Bugs

*r Creating a complex number from Infinity no longer crashes and returns 'I * Infinity'