additional capabilities (and an api change)

calculate skewness and kurtosis and make sum more robust.
Use floating point values wherever possible to avoid implicit conversions of ints.

This PR uses the method described in http://prod.sandia.gov/techlib/access-control.cgi/2008/086212.pdf

I took the liberty to introduce an api change for snapshotting, but I was too lazy to fix the tests. Feel free to add comments in the PR.