Use AMS Euler fonts for math

Provides a setup for using the AMS Euler family of fonts for mathematics in LaTeX documents. "The underlying philosophy of Zapf's Euler design was to capture the flavour of mathematics as it might be written by a mathematician with excellent handwriting." The euler package is based on Knuth's macros for the book 'Concrete Mathematics'. The text fonts for the Concrete book are supported by the beton package.