Patch for 'normalize' that crashed with empty set

When a function is called on an empty serie, the function 'normalize'
encoutered an error.
Now it raises a custom error that is tracked by the function 'evaluateTokens'
and returns []
adding test function for normalize()
Merge branch 'f_normalize'

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

Conflicts:
	webapp/graphite/render/evaluator.py
	webapp/graphite/render/functions.py