Add a new continuous colour/fill scale using colorbrewer colours

It fetches 6 colours from a colorbrewer palette and feeds them to gradientn. It gives very nice results even if the palettes were not intended for that use originally.
Add the American spelling synonym scale_color_brewerc()
Expand documentation of scale_brewer

As per Hadley's request: "add a few words to the main doc mentioning
that it does both continuous and discrete variables""
Further expand the description of scale_brewer

Give more precisions regarding the intent of the original colours and how
ggplot2 modifies them (in the continuous case).
Break lines at 80 characters
Change name from brewerc to distiller
Add examples for continuous brewer palettes
Merge branch 'feature/brewer-continuous'