add command line option to get user number statistics

please review @karlitschek @DeepDiver1975 @icewind1991 @ringmaster  or others.

Would be great if someone could test it on IIS, too.

Output could look like this (e.g. without ldap the ldap line won't be there)
![userreport](https://f.cloud.github.com/assets/2184312/1868075/d3ace392-7860-11e3-8b98-6de3233102bf.png)


cc @MTRichards