Visual regression tests

[CiaranOMara]

I want to create a pull request for visual regression tests. So far, I've seen two variants, the one in Gadfly.jl, and various uses of VisualRegressionTests.jl. Do you have preferences for how to manage visual regression tests?

[Mattriks]

Also comments in #334. And currently the Compose docs are another visual test, and devs should be viewing them with any new PR.

[bjarthur]

historically compose and gadlfy have been so tightly intertwined (they used to be one and the same before compose was split out) that gadfly's regression tests served double duty as testing compose as well. i think it's fine as is, but if someone wants to tackle this that'd be great. easiest would be to duplicate gadfly's comparison script. better would be to split it out into its own repo.