Use plot recipe instead of pyplot

[odow]

Pyplot is a pretty heavy dependency.

An alternative is to write a plotting recipe: https://github.com/JuliaPlots/RecipesBase.jl

This lets you have a very light dependency, and users can use whichever Plots.jl backed they like to plot.

Here's an example in SDDP.jl: https://github.com/odow/SDDP.jl/blob/master/src/visualization/publication_plot.jl