BUG: normalize rolling_window() weights, remove scikits-timeseries dependency for testing

This closes #7618 and removes the dependency on scikits-timeseries for testing rolling_window functions. Scikits-timeseries has not been maintained since 2009, and I had difficulty simply installing it on my system. Instead of using scikits-timeseries to calculate the various rolling moments on randomly generated data, the tests use pre-calculated data.