Fix colour='NA': change output from 'none' to 'transparent'
Ran roxygen2::roxygenise()
Improve tests
Add colour=NA to polygon and ribbon; and test
geom_polygon default fill to 'grey20'
Increment version number to 0.5.9
Merge pull request #140 from ropensci/pedro-default-colours

ggplot: colours: geom_polygon() defaults, and transparency