Great idea, looks like, but I do hope you're aware there are many of these around.

http://www.buxfer.com/

http://www.splitwise.com/

https://wesplit.it/

http://www.billsarein.com/

https://www.paydivvy.com/

https://www.billmonk.com/

https://splitabill.com/

http://flatmin.com/

http://expensure.com/

I personally use the first two, in my roommate / office situation respectively. They work great, frankly. I have very few complaints about either.

If you can do it better, more power to you, but as with any crowded space, I have to ask, because I'm genuinely interested—why did you choose to start from scratch?