Improve thumbs script

Summary: I changed the script so that when I call it with no arguments, it just Does The Right Thing (makes original and thumbs).

Test Plan: - Run script on an images directory, it should automatically resize and make thumbs

Reviewers: alexrodrigues

Reviewed By: alexrodrigues

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D14419