Before 1.0 release:
- Fix axis_canvas()
- Add as_grob() function for gList
- null devices should make temporary files and clean up after exit - may not be technically possible

Things to fix or improve
=============================
- more regression tests are needed.
- write vignette on themes

Ideas for further development
=============================
- make a theme for individual plots and one for grouped plots (and one for faceted plots?), make sure that all default settings work well together.
