swap blues in Zissou
Convert discrete palette to a colorRampPalette function.

n is no longer required, as the behaviour of the return value is now
consistent with other palette functions (e.g. heat.colors(),  etc)
Depends on R >= 3.0, and update man page to remove n
Merge pull request #1 from richardsc/master

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