Initial commit of geom-arrow, based on geom-field from  https://github.com/hadley/ggplot2/wiki/Creating-a-new-geom
geom arrow actually runs, though it doesn't do much useful...
much more funcitonal, refactored to just use segmentsGrob directly, still need arrow
eliminated amateurish debug code
geom_field: arrows work, but no alpha channel
rename geom_field to geom_arrow & comment