Partial derivative function of fuzzy membership functions

I'm working on some back-propagation algorithms that require me to calculate the partial derivative of fuzzy membership functions, with respect to their various parameters.  I thought this would be useful functionality to add to the excellent scikit-fuzzy module, under the fuzzymath operations umbrella.  This commit is an initial preliminary attempt at a potential function that will (hopefully) ultimately produce the partial derivative of all the fuzzy membership functions covered in 'generatemf.py'.  As a first attempt, I've only addressed the 'gaussmf' membership function,  After receiving feedback, the intention would be to add more membership functions, under the 'elifs'.  This is my first pull request to a public project.  I've tried to match style and maintain good python practice.  Be gentle. 
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