Tidy up main vignette file

* Fit lines into 80 chars
* Change item lists (bullets) so that they are correctly rendered in Rstudio
Change CRS parameters to an EPSG code
Update vignette intro
Remove chunk hkk-suuralue4
Remove ggmap plotting from chunk peruspiiri
Add horizontal dividers
Remove chunk MML_provinces
Remove chunk road-accident-density
Update md vignette
Add caches to gitignore