Differentiate polygamma

[fredRos]

I would like to add support for differentiating the polygamma function, see JuliaDiff/ForwardDiff.jl#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