added two functions to functions.py

I've added two functions 
1)  stddevSeries will generate the standard deviation in a series much like averageSeries will
2)  timeStack  will take a single series and slice in ( using multiple timeShifts) into multiple series. 

The combination here will allow you to do something like create the average shape of some data over the last 14 days.  And then add some bands based on stddev.