function zScore(x, mean, standardDeviation) {
    return (x - mean) / standardDeviation;
}
The [Z-Score, or Standard Score](http://en.wikipedia.org/wiki/Standard_score).

The standard score is the number of standard deviations an observation
or datum is above or below the mean. Thus, a positive standard score
represents a datum above the mean, while a negative standard score
represents a datum below the mean. It is a dimensionless quantity
obtained by subtracting the population mean from an individual raw
score and then dividing the difference by the population standard
deviation.

The z-score is only defined if one knows the population parameters;
if one only has a sample set, then the analogous computation with
sample mean and sample standard deviation yields the
Student's t-statistic.

@param {number} x
@param {number} mean
@param {number} standardDeviation
@return {number} z score
@example
zScore(78, 80, 5); // => -0.4