Quick approximation for map projections

Instead of reprojecting everything, we just set the aspect ratio
of the plot to the ratio between the length of one degree of lat
over one degree of lon.
Fix bug in documentation

The latest version of roxygen figures out S3 methods on its own
Fix coding style and spelling for new coord_quickmap
Add NEWS bit for coord_quickmap
Merge branch 'quickmap' of github.com:jiho/ggplot2 into jiho-quickmap