Merged in bgyori/unigen/fix_exit (pull request #2)
In some environments exit itself doesn't work (I ran into an error due to this), I changed these to sys.exit, see e.g. https://stackoverflow.com/questions/6501121/difference-between-exit-and-sys-exit-in-python
exit
sys.exit
In some environments
exititself doesn't work (I ran into an error due to this), I changed these tosys.exit, see e.g. https://stackoverflow.com/questions/6501121/difference-between-exit-and-sys-exit-in-python