Imported ressources (tura, gewata, figures) from rasta + rforge download page
Second pass on the tutorial, to change what needed to be changed. Test Data are now downloaded from github diectly. Remains to re-work the last 'taravao'section (data ordered)
Added ref file and changed all download.file() commands to work under windows
Re-added Gewata dataset after cleaning history of all tifs (bad commit with big files)