KernelDensity.jl support

[simonbyrne]

I've moved the support for kernel density estimators (kde) functionality out of StatsBase.jl and into a new package KernelDensity.jl. I'd be keen update the support for the Gadfly package, however I don't really understand the Gadfly internals sufficiently well to do this on my own.

There is currently some "glue" code for interacting with Winston and PyPlots, which is automatically loaded if the relevant package is detected. I would be happy to include Gadfly support as well if you don't want to add another dependency to this package.

cf JuliaStats/KernelDensity.jl#5

[dcjones]

Thanks for reminding me of this. I think I'll just make KernelDensity a dependency for now.

There are several packages that should eventually be made optional (Datetime, Hexagon, Loess, KernelDensity), and I'd like to have some consistent mechanism to deal with that. Maybe conditional modules, if that becomes a thing.