Tweaking ICE undo feature

@jbau & @VikParuchuri - Here's the PR to get the undo button to work in ICE (FINALLY!) If you could take a look at the code, I'd appreciate it!

@gbruhns  - wanna look at this on stage and confirm it works? (https://stage.class.stanford.edu/courses/SU-VPOL/ORA-ICE/2013/courseware/e8bd2c60ae934eacb55dfae0aa030d34/23917561ac2a4b409d5df2dfc4155c0f/)