IMHO as a long term member of the software engineering community, I think that you're covered and can reasonably get out. Your previous employer is greedy, and wants more of a good thing.
Since you're part of a development effort and may have a body of unique knowledge, it would be a good idea to offer to help them transition in a new person to replace you, and to be willing to answer questions they might have on a contract basis after you've left. This is really up to you, though. You're not required to do this, and if they get mean or greedy about it you should definitely cut them off.
This is no more than we can expect from employers these days. We get our two weeks severance and we're out the door, and we consider ourselves lucky to get that. You may want to consider any personal relationship you have with the management, but professionally speaking, I think you're doing just fine.