minor changes in related work paragraphs
added goals and assumptions paragraphs
added motivation for FAST choice, corrected other sentences and typos
added ORB paragraph
minor correction of senteces and typos
changed signature to IP-BGS method apply(), clustering code moved to
ObjectClustering class, object sample thumb saved in save(), added code
for debug/test
added fg/bg IP classification test, added clustering test, added
pick-and-show mode to ClassViewer, realigned old tests