derivatives of Max() and Min() using Heaviside()

modified:   sympy/functions/elementary/miscellaneous.py
modified:   sympy/functions/elementary/tests/test_miscellaneous.py

* added fdiff for Min() and Max()
* added diff Min() and Max()
* implement simple tests of Max().diff and Min().diff
* use ArgumentIndexError for assertions in fdiff
Merge pull request #1707 from zanzibar7/Max_and_Min_Derivatives

derivatives of Max() and Min() using Heaviside()