Use Plots.jl for plotting

[fairbrot]

Using the Plots.jl package would remove the need to define optional package dependencies and define plotting functions for each plotting back end. This would open up many new plotting back-ends (such as Plotly) and only the skeleton package RecipesBase.jl would need to be added to REQUIRE.

[kskyten]

This seems to be implemented in the plot branch. Can it be merged? I have a recipe for 2D GPs.

[fairbrot]

Could you make a pull request on the plotting branch? Once this is added to the plot branch, I'll delete the old plotting functions in master and merge.