Hi,
i wanted to do this on my own but as i dont have much time lately and other people are probably more qualified than me anyway 
i wanted to propose to include the TrueSkill Algorithm by microsoft into the mam.
A PHP Implementation of this can be found here:
https://github.com/moserware/PHPSkills/
More about the Math behind it can be found here: (ordered from less to more detailled)
http://research.microsoft.com/en-us/projects…ll/details.aspx
http://www.moserware.com/2010/03/computing-your-skill.html
https://dl.dropboxusercontent.com/u/1083108/Moserware/Skill/The Math Behind TrueSkill.pdf
and an online calculator: http://research.microsoft.com/en-us/projects…alculators.aspx
Would really do it myself but cant find any time soon so probably has to wait till summer till i can get to it. However if i find some time before i will definitely look into this more.