Make sliders possible for logspace

[gnestor]

via jupyter/notebook#183:

@shashi wrote:

as discussed in JuliaGizmos/Interact.jl#2 Currently the sliders work only for linearly increasing ranges. But it would be nice to have sliders work for logspace(-3, +3).

[jasongrout]

Thanks! We're tracking the conversation about log-scale sliders at #719.