Update to the CharacteristicClasses package

I have added an implementation of the algorithm described in:
An Algorithm to Compute the Topological Euler Characteristic, Chern-Schwartz-MacPherson Class and Segre Class of Projective Varieties (Martin Helmer), arXiv:1402.2930 [math.AG]

This algorithm complements the current algorithms used by providing improved performance in many cases, especially when working over a finite field. 

The original author of the CharacteristicClasses package, Christine Jost, recommended I add my implementation to the package. As we discussed via email I have made the new algorithm the default over finite fields, while Christine's algorithm remain's the default option over the rational numbers. I have also added to the documentation to reflect the changes. 

Please comment with any questions, or if there are any problems with my changes. 

Thank you, 

Martin