Implement more benchmark functions

[halfdan]

The library definitely needs more benchmark functions for evaluation.

A nice compilation of functions can be found at

• http://www.zsd.ict.pwr.wroc.pl/files/docs/functions.pdf
• http://www.geatbx.com/docu/fcnindex-01.html
• http://www.sfu.ca/~ssurjano/optimization.html