enable continuous integration via craigcitro/r-travis

[krlmlr]

• install binary R packages where available
• supply .gitignore

[krlmlr]

@hadley: This might help addressing the issues raised in your earlier comment. Let me know if I can help by enhancing this PR.

All in all, I think people will accept visual changes if they are documented appropriately. tikzDevice has visual integration tests that compare results at the pixel level. I see save_vtest in your code, and the vtest package. What do I need to do to activate the visual tests? What is so onerous about them (or about testing ggplot2 in general)?

I haven't looked into vtest yet, but I would compare images at the following levels:

• PDF output, to check that results are identical
• Output of tikzDevice, to check if individual grobs have moved/changed
• PNG output, to see how severe the change is visually

GitHub has a nice GUI for comparing images.

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