Differentiate polygamma

I would like to add support for differentiating the `polygamma` function, see https://github.com/JuliaDiff/ForwardDiff.jl/issues/196. Do you seen any difficulties? I know that one needs to access the first arg to `polygamma`, so I can't just add a single line to `differentiate.jl`. It seems the Bessel functions have the same structure. The comment in https://github.com/johnmyleswhite/Calculus.jl/blob/master/src/differentiate.jl#L264 even mentions the digamma function as a function to add. So is there any stumbling block? I'd be willing to send a PR with a little guidance