Make treatment of sample size consistently refer to minutes

Maybe not a permanent fix, depending on any changes to the required movingAverage, but this addresses the issue raised in https://github.com/wayfair/Graphite-Tattle/issues/34.

In most of the code, including the getData function in Check.php actually responsible for checking a metric's state, sample size is treated as referring to minutes, whereas constructTarget treated sample size as referring to a number of samples. 

These changes make Tattle consistent in treating sample size as a number of minutes.