213 attempted solution

I tried to solve #213, the result is compatible with the mathematica result even if some ratios of gamma functions are not simplified properly
For complete MMA compatibility we should probably fold both forms into `Gamma`. It looks like doing proper conversion to and from SymPy might be tricky in this case though.

Maybe we should extend `SympyFunction`/`_MPMathFunction` so that it's possible to multiplex several SymPy functions onto one Mathics function with rules?