Just out of curiosity...You say there was no written contract at the start? If you wrote everything (or at least the majority of the code) and there's no written IP assignment, then you could conceivably _leave with the product_. It's yours, not theirs. Maybe. At least, the IP is encumbered by that fact.

I'm not saying leave w/ the IP, but you may in fact have a LOT more leverage here than you thing. You need to talk to a lawyer ASAP.