Allow to reverse the order of colours in a brewer palette

[jiho]

Use the same argument that hue_pal uses: direction

[hadley]

Could you please merge/rebase and add a bullet to NEWS?

[jiho]

Done, and squashed everything into one commit.

[jiho]

I forgot to write the documentation before. Sorry. check() still does not pass on my machine because of problems in creating the PDF documentation. However there is no more info and I don't really know what to fix. Hopefully that's a glitch of my machine and it'll pass on Travis.

[jiho]

It passed! It'll be great if you can merge it before the release so that I can depend on it in ggplot2.

[hadley]

For future reference, I prefer to use return() only for early returns

[jiho]

OK. I thought it was good practice to always have an explicit return()(it's certainly less confusing for newcomers/students in my experience).

[hadley]

Thanks!

[hadley]

BTW if you have a little more time, I'd love a PR with some tests :)

[jiho]

What would you want me to test in this particular instance? The order of colors returned? The values of colors returned?

[jiho]

Thanks for merging it anyway, I'll get to work on the ggplot end now!