adding ddof parameter back into std and var
Issue #934

I just got bit by this, and I think it's a pretty serious bug. If someone will merge this, I will write some tests.
 Just curious what your application is that needs population std measures?
 I need sample std measures for calculating standardized (internally studentized) residuals in a linear model.
 If that's the case shouldn't the default pandas behavior be correct?
 No. I need sample statistics. In any event, this is a bug.
 Oh, you need ddof > 1 in your use. Got it.
 Yeah ddof = model degress of freedom in this case.
 +1 Need this as well 
 merged in and tested. need to update the docstrings. someday
