Add a "strict mode" to cronbach_alpha

[agarie]

According to Claudio in clbustos/statsample#11, tests in which any vector of responses for a specific item has zero variance should be considered a mistake, at least in a strict_mode.

So: implement a strict_mode flag that raises a Statsample::ZeroVarianceVectorException (feel free to choose a better name) if any of the vectors in cronbach_alpha has zero variance.