Add feature to generate objects (ie. alpha-masked objects from png imgs)

[tpham3783]

to aid AI training for object localization.

To generate a sample dataset, run:

run.py -c 60 -b 100 --text_color black -num -let -id images -w 200 -w 300 -wd 600 -k 20 -rk -d 3 -na 3 -bl 2 -rbl -i /tmp/alpha.txt -it object

where /tmp/alpha.txt is a listing of png files like the following:

/tmp/car.png /tmp/person.png /tmp/banana.png