Add a function "callbackToNode"

Doc and example in the code
Use of function sort instead of sorted
Function that exacerbate the extrema of a graph
Doc and improvement for exacerbate function

Now, it can have several function in argument
Function to draw an area with the min and max

If a graph has its width smaller than the number of measures, it calls
a consolidate function to know what is the better point to draw.
This function draw the min and max points for each pixel to consolidate
and fill the area between the curves.
Patch for the constantLine function

When we wanted to draw a graph with just a constantLine,
it raised a ZeroDivisionError.
This patch corrects this error.
adding test function for constantLine()
adding groupByNodes() function, same as groupByNode() w/ several nodes
Merge branch 'f_minmax'

* f_minmax:
  Function to draw an area with the min and max
  Doc and improvement for exacerbate function
  Function that exacerbate the extrema of a graph
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
Merge branch 'f_consowhisper'

* f_consowhisper:
  consistency between "aggregationMethod" and "consolidateFunc" names
  pass data-guessed consolidation method through MultiReader
  Send data-guessed consolidation function through pickle
  Take the aggregation method of whisper just for min or max
  Use the consolidate function for min or max series
Merge branch 'f_normalize'

* f_normalize:
  adding test function for normalize()
  Patch for 'normalize' that crashed with empty set
Merge branch 'f_constantline'

* f_constantline:
  adding test function for constantLine()
  Patch for the constantLine function
Merge branch 'f_groupbynodes'

* f_groupbynodes:
  adding groupByNodes() function, same as groupByNode() w/ several nodes

Conflicts:
	webapp/graphite/render/functions.py
working around issue #712
Merge branch 'wa_carbonlink_broken'

* wa_carbonlink_broken:
  working around issue #712