Further improving ggplot2 theme

[greimel]

First steps were taken in #30. But there is more to do.

• Colors: ggplot2 uses the HCL color scheme according to https://stackoverflow.com/a/8197706. This would probably have to implemented in JuliaGraphics/ColorSchemes.jl and then used here.
• Minor grids and minor ticks

[isentropic]

minor grids and ticks support is greatly improved in pyplot, would you please try it with pyplot?