added section on plotting

[pfitzseb]

PyPlot is described fairly detailed, but since I don't really use
Winston and Gadfly I've only added the most basic stuff for them.

Since I couldn't build the site I don't know if my changes work as is, but I hope they will.

[samuelcolvin]

Great to have someone else's input, thanks for contributing. Plotting is something that's definitely needed.

A few comments:

• It makes sense to use import rather than using in this context to keep the namespaces separate, but in general using is more user friendly. We should at least explain this, and perhaps use user in one case to demonstrate?
• More generally this would be the first time we've mentioned import or using in the page, we should probably provide a link to the docs or a little explanation on what's going on. On second thoughts, I will add a using_Pkg.jl example above this to show how to add packages, maybe this chat can go there.
• I'm trying to keep all the examples working "out of the box" - so if you hit download and run the file, something good happens. For that reason the paths should probably just be file names so they drop into the local directory. Currently the script crashes as obviously i don't have a path/to/save/to/ folder.
• There's some funky template rendering that means comments are rendered as a subset of markdown so links work as links. Might as well continue to follow that, see the other examples.

All these changes are minor and in general this looks great. Either I can accept the pull request then make the tweaks myself or you can add another commit to fix some or all of them. Up to you?

[pfitzseb]

I've fixed the latter two points, but the using/import/Pkg....-stuff warrants an additional example, in my opinion (I guess I could find some time for that later today).

[samuelcolvin]

Great I'll add this and build the site as soon as I get a chance.