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
Updated documentation with latest version of roxygen
Fix coding style for brewer scales

Add spaces around `=`
Avoid confusion in warning messages

The actual function call from which the warning is emitted might not be that helpful to the casual user.
Simplify documentation of brewer scales
Add note about the number of interpolated colours
Add NEWS bit for distiller scales
Merge pull request #925 from jiho/feature/brewer-continuous

Rebased version of continuous brewer color scales