I just published the plugin dzetsaka : classification tool (github/qgis) on Qgis plugin server. This is a very simple plugin to make a fast but powerful classification. I used it for making numerous classifications of SPOT 5 scenes in the French Guiana Amazon forest.
A project with samples (an old map (map.tif) and ROI (train.shp)) is available on the docs branch. You can download these files directly here.