implement new MeanVariance and Variance

Simplified Variance by removing the input mean from the sig, and
implemented a MeanVariance function which calculates the corrected
two-pass corrected sample variance.  It also calculates the mean, so it
returns that as well, which should save some calculation under most
cases.
simplify sig to StdDev

Removed the mean from the standard deviation calculation.
Add MeanStdDev function

This function is used to calculate both the mean and standard
deviation, which should save some calculation under most circumstances.
Simplify covariance sig

Changed covariance to remove the need to supply the means.  Also
implemented the corrected two-pass method to estimate the covariance.
Simplify Correlation Sig

This used similar code as the Covariance function.  Also, the example
was incorrect (verified via MATLAB) and had a line
`meanY := Mean(x, w)`

Which demonstrates the problem of calculating the weighted mean and
standard deviation externally.
Simplify Skew Sig

Simplify the Skew interface to remove dependence on mean and standard
deviation calculation.
Simplify ExKurtosis Sig

Remove mean and std from the input to ExKurtosis.
Simplify Moment Sig

Simplify the moment function to no longer take a mean, and implement
tests.
Rename MomentAt -> MomentAbout

Change the name of MomentAt to MomentAbout, as discussed on gonum-dev.
fix dist

Forgot to run go test on stat/test.  Fixed Moment calls.
Add benchmarks for moment statistics

Added benchmarks for various sizes of the moment measuring statistics,
utilizing both weighted and unweighted paths.
use new location for go cover tool
use two pass correction in correlation
use two pass correction in covariance
change MomentAbout help text
run go fmt

forgot about it
comment for source of correlation algo
comment for source of covariance algo
remove unneeded ExKurtosis comment
remove unneeded Skew comment