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
Document all themes together

Add a slightly more detailed description of the themes and a few words about their intent.
Add examples to ease comparison
Slightly alter `theme_light` to make it less redundant with others
Document themes more homogeneously

Make all themes symmetrically reference the general ggtheme documentation
Document theme_grey and theme_gray
Merge pull request #925 from jiho/feature/brewer-continuous

Rebased version of continuous brewer color scales
Merge pull request #933 from jiho/theme_doc

Document all themes together
Allow to reverse the order of colours in a brewer palette

Use the same argument that scale_*_hue uses: direction
Make scale_*_distiller work with the direction argument