Include Standard Deviation accumulate function

Hi guys,
I think we should add the standard deviation to accumulate functions (even excel has it I went through the Jira there is no similar request, although people are asking about it on the mailing list - so why not make their lives easier?
http://lists.jboss.org/pipermail/rules-users/2011-April/019494.html

It can nicely reside here with others
https://github.com/droolsjbpm/drools/tree/master/drools-core/src/main/java/org/drools/base/accumulators

Edson's article on how to add custome ones
http://blog.athico.com/2007/06/accumulate-functions-edson-tirelli.html

If it sounds feasible assign back to me and I can try to contribute,
Cheers.