hexbin color key title from "Color" to "Count"
hexbin color key title from "Color" to "Count"

This is copied from the `histogram2d` code. I believe it makes sense for `hexbin` as well.
http://gadflyjl.org/geom_histogram2d.html
http://gadflyjl.org/geom_hexbin.html
Roger-luo/Gadfly.jl,tbreloff/Gadfly.jl,joehuchette/Gadfly.jl,maximsch2/Gadfly.jl,peter1000/Gadfly.jl,zhmz90/Gadfly.jl,spencerlyon2/Gadfly.jl,JuliaPackageMirrors/Gadfly.jl,Godisemo/Gadfly.jl,jiahao/Gadfly.jl,kshramt/Gadfly.jl,dcjones/Gadfly.jl