function sampleCorrelation(x, y) {
    const cov = sampleCovariance(x, y);
    const xstd = sampleStandardDeviation(x);
    const ystd = sampleStandardDeviation(y);

    return cov / xstd / ystd;
}
The [correlation](http://en.wikipedia.org/wiki/Correlation_and_dependence) is
a measure of how correlated two datasets are, between -1 and 1

@param {Array<number>} x first input
@param {Array<number>} y second input
@returns {number} sample correlation
@example
sampleCorrelation([1, 2, 3, 4, 5, 6], [2, 2, 3, 4, 5, 60]).toFixed(2);
// => '0.69'