Update `n` functionality using `colorRampPalette`, and switch order of blues in "Zissou"

Hi! Nice work putting together this package. I'm an oceanographer (who uses R), and so of course as soon as I saw a link to your repo on Twitter I had to try the "Zissou" palette. I ended up forking your repo, to swap the order of the blues (so it makes more sense for plotting data), and I added `colorRampPalette` functionality so that the number of colors can be specified easily, beyond the original color levels.
![zissou_adcp](https://f.cloud.github.com/assets/233584/2464473/601bc150-af97-11e3-8048-c17790b740cf.png)
Feel free to follow me @ClarkGRichards to see the ensuing discussion and excitement about trying to use "Zissou" in an oceanographic publication!