Use the consolidate function for min or max series

When the function maxSeries or minSeries is called, the consolidate function
'min' or 'max' was added to the TimeSeries created in order to have a better graph.

Also, the aggregation function of the whisper serie is retrieved, so
the TimeSerie could be created with the good consolidate function
Take the aggregation method of whisper just for min or max
Send data-guessed consolidation function through pickle
pass data-guessed consolidation method through MultiReader
consistency between "aggregationMethod" and "consolidateFunc" names

first one is whisper's name, second one is graphite's